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Common folklore dictates that the Surreals were discovered by John Conway as a lark while studying game theory in the early 1970's, and popularized by Donald Knuth in his 1974 novella.

Wikipedia disagrees; the page claims that Norman Alling gave a construction in 1962 which modified the notion of Hahn series fields to realize a field structure on Hausdorff's $eta_alpha$-sets, and that this construction yields the Surreals when considered over all ordinals -- the article pretty explicitly characterizes Conway's construction as a popularized rediscovery more than 10 years later (!).

The given reference links to a paper from 1962 (received by editors in 1960) which seems to fit the bill, however this paper gives no indication that Alling considered his construction extended to all ordinals nor does it contain mention of proper classes that I can find.

Is there any evidence indicating that Alling considered a proper class sized version of his construction almost a decade before Conway did?

Alling published a book in 1987 which constructs the Surreals in exactly this manner*, but this is (of course) more than a decade after Conway's construction was popularized. Was this construction known to Alling earlier and only written up in book form when he realized there was a more widespread interest, or were proper-class considerations in this arena genuinely not in play before Conway?

*This is incorrect, thanks to Philip Ehrlich for pointing out my mistake. Alling's 1987 book doesn't construct the surreals as modified formal power series in the fashion of his 1962 paper; the methods involved are more subtle. On January 3, 1983 and December 20, 1982, Norman Alling and Philip Ehrlich respectively submitted papers which constructed isomorphic copies of the surreals using modified versions of the constructions in Allings 1962 paper, and these submissions eventually lead to the publication of two papers to this effect. The 1987 book mentioned above contains expansions of these works in sections 4.02 and 4.03, respectively. (see Philip's excellent answer below for a better description of events)

Norman Alling's Conway's field of surreal numbers (1985) gives full credit to Conway:

Conway introduced the Field No of numbers, which Knuth has called
the surreal numbers. No is a proper class and a real-closed field,
with a very high level of density, which can be described by extending
Hausdorff's $eta_xi$ condition. In this paper the author applies a
century of research on ordered sets, groups, and fields to the study
of No.

References to Alling's own 1962 paper appear only in passing, on page 373 and 377–378. The 1987 book referred to in the OP follows up on this 1985 exposition, which makes it clear that Alling in no way claims independence from Conway.

If one thinks of the surreal numbers as just a proper-class sized saturated real-closed field, then I think Alling deserves much of the credit for the discovery or invention. He didn't deal with the proper-class sized case, but his hypotheses do cover the case of strongly inaccessible cardinals. A strongly inaccessible cardinal can be viewed as class-sized simply by cutting off the (cumulative hierarchy view of the) universe of sets one level after that cardinal. So I'm inclined to view the passage to proper-class size as no big deal.

But the field of surreal numbers has an important additional structure, namely what Conway calls "birthdays". In other words, this field has a very natural construction in terms of transfinitely iterated filling of Dedekind cuts. As far as I know, this construction and the fact that it produces a saturated real-closed field are entirely due to Conway.

Let me begin by adding my voice to those who have said that Conway is the sole originator of the theory of surreal numbers, the reasons given by Andreas being critical. Moreover, based on my conversations and correspondence with Norman (Alling), with whom I did the following joint work on the surreals in the 1980s, there is no question he would concur.

(i) An Alternative Construction of Conway's Surreal Numbers, C.R. Math. Rep. Acad. Sci. Canada VIII (1986), pp. 241-46.

(ii) An Abstract Characterization of a Full Class of Surreal Numbers, C.R. Math. Rep. Acad. Sci. Canada VIII (1986), pp. 303-8.

(iii) Sections 4.02 and 4.03 of Norman Alling’s Foundations of Analysis Over Surreal Number Fields, North-Holland Publishing Co., Amsterdam, 1987.

My principal reason for writing this answer is to help clarify the relation between Conway's (1976) work, Norman’s work of 1962, and some work Norman and I did independently in the early 1980s making use of Norman’s work of 1962, and to correct the mistaken characterization of the construction of the surreals contained in Norman’s book as described by Alec in his question.

In his paper of 1962, Norman proves the existence of a real-closed field that is an $eta_alpha$-set of power $aleph_alpha$, whenever $aleph_alpha$ is regular and satisfies another natural set-theoretic condition, thereby providing an affirmative answer to a question of Erdös, Gillman and Henriksen that arose from their work on rings of continuous functions. To prove the result, Norman employed a construction that is a marriage of Hahn’s (1907) celebrated construction of non-Archimedean ordered fields and Hausdorff’s (1907) equally celebrated construction of $eta_alpha$-sets obtained from transfinite sequences of 0s and 1s. In my opinion, Norman’s result is one of the genuinely important results of the 20th-century theory of ordered algebraic systems.

On January 3, 1983 and December 20, 1982, Norman and I respectively submitted papers for publication in which we show that an isomorphic copy of Conway’s ordered field $mathbfNo$ could be obtained using Norman’s just-mentioned construction or simple variations thereof. Norman did this via the union of a chain $K_alpha + 1$, $alpha in mathbfOn$, of real-closed fields that are $eta_alpha + 1$-sets (constructed with minor modifications as in his (1962)) and I did it more simply using Hahn’s construction in conjunction with Custa-Dutarti’s construction of successively filling in cuts (Algebra Ordinal, Rev. Acad. Ci Madrid 48 (1954), pp. 103-145), a construction Harzheim had shown leads to various $eta_alpha$-sets at various levels of recursion (Beiträge zur Theorie der Ordnungstypen, lnsbesondere der $eta_alpha$-Mengen, Math. Ann. 154 (1964), pp. 116-134). In our respective papers, we also introduced analogous constructions for families of isomorphic copies of distinguished subfields of $bfNo$ that are real-closed fields that are $eta_alpha$-sets. Norman’s paper appeared as Conway’s Field of Surreal Numbers, Trans. Am. Math Soc. 287, (1985), pp. 365-386, and a portion of my paper, which was initially submitted to Fund. Math., eventually appeared six years later as An Alternative Construction of Conway's Ordered Field $bfNo$, Algebra Universalis, 25 (1988), pp. 7-16. Another portion of that work appeared in my Absolutely Saturated Models, Fund. Math., 133 (1989), pp. 39-46. While I was waiting to hear back from the journal, I sent my paper to Norman, who I did not know at the time and who informed me that his paper (which I was unaware of) had been accepted for publication. Nevertheless, he believed my paper, which differed from his in various ways (including an emphasis on model theoretic connections, and the inclusion of the Custa-Dutarti cut construction which he was not familiar with), should be published, and he proposed we carry out joint work, which led to (i)-(iii) above. (i) is a treatment of the construction of the surreals based on the Custa-Dutarti cut construction, (ii) provides an axiomatiization of the surreals including its birthday structure, and (iii) is two subsections of Norman's book containing expansions of the just-said works.

As my characterization of (iii) suggests, Alec is mistaken when he asserts that in his (1987) Alling constructs the surreals as a field of formal power series (using techniques from his (1962)). What is true is that long after Norman and I introduce the surreals in that work using the Cuesta Dutari cut construction (pp. 121-127), with sums and products defined à la Conway, Norman points out (see pp. 246-247) that $bfNo$ so constructed is isomorphic to a distinguished field of formal power series. It is worth noting that while Conway was not familiar with the historical background that Norman and I drew attention to in our papers from the 1980s, Conway was aware of the relation between $bfNo$ and distinguished fields of formal power series (as is evident from Theorem 21 and the subsequent remarks from in his monograph On Numbers and Games). A simple proof of that relationship making use of $bfNo$’s simplicity hierarchy is the proof of Theorem 16, p. 1249 of the present author’s Number Systems with Simplicity Hierarchies: A Generalization of Conway’s Theory of Surreal Numbers, J. of Sym. Log. 66 (2001), pp. 1231-1258.