I have some new information to respond to increment's post with, so here we go…
A March date was already around in earlier looks at the history of the game, and what I saw was consistent with that.
Can you be more specific than “earlier looks”? Where was it stated prior to
Playing at the World that
Chainmail was published in March besides the 2006 Acaeum forum post?
My date was, following Zenopus's lead above, based in no small part on the earliest advertisements for Guidon Games, which appeared in the April International Wargamer and a few other contemporary places. But that has to be understood in terms of how the IW production schedule operated at the time, and who got which issues when. The short story is that I concluded that the March and April IWs were produced and shipped simultaneously - so effectively, there was no issue shipped in March –
Here’s the March issue (for research purposes only):
International-Wargamer-March-1971.pdf
The issue notes that it is “commemorating IFW’s Fifth Anniversary – March 1966 – March 1971. It makes no mention of being published two months late. It also contains no ad for
Chainmail, unlike the April issue.
and that IW production had a long lead-time, maybe up to six weeks between layout and issues being in people's hands. There are good data points to suggest that the March and April IWs were jointly in subscribers' hand by the third or so week of April. Note that the April IW ad lists Chainmail as available, but suggests that Alex and Dunkirk were scheduled for an April 30 release - something you would not bother to mention if you expected your mail-order ad to be seen like April 18th. But these are ultimately indirect data points, and to be absolutely clear, they don't rule out an April "release" date for Chainmail - if you're counting from when just anyone could have ordered it, it wouldn't be until after they saw an ad.
Given that the March issue has no ad for
Chainmail and the April issue does, I don’t see the relevance of when they actually shipped. Obviously it was planned for the
Chainmail ad to appear in the April issue but not the March issue, which is good evidence that
Chainmail was not available in March--regardless of when the two issues actually shipped.
Dave Arneson was not "just anyone", and that he was surely aware of Chainmail before April.
While it may be true that Arneson “was surely aware of
Chainmail before April,” awareness of
Chainmail does not mean that Gygax had sent him a copy of it--which is what the thesis in
Playing at the World that Arneson “design[ed] his new game around the fantasy elements of the just-released
Chainmail” [PatW 65] would require. You stated in
Playing at the World that:
this study anchors all major events, dates and sequences related to the history of Dungeons & Dragons on contemporary sources, which is to say sources printed within a year or so of the events in question-preferably far closer.
Yet, you seem to have no tangible evidence of the March publication date for
Chainmail and no tangible evidence that Gygax sent Arneson a copy of
Chainmail before it was published. In fact, on page 41 of
Playing at the World you state:
Quite late in the development of Chainmail, Gygax decided to furnish the game with a supplement dealing with a very different sort of combat. As he offhandedly reported to Wargamer's Newsletter in early 1971:
We are also planning to write up rules for Tolkien fantasy games, using LGTSA Medieval Miniatures rules as the basic starting point. […]
Yet, we see Arneson’s Blackmoor—complete with Fantasy elements—already being described to Gygax in a March 1971 letter. How is it possible that Gygax was “planning to write up rules for Tolkien fantasy games” in “early 1971,” with those rules having already been written, tested, printed, and published in time for Arneson to create Blackmoor with the published
Chainmail booklet and write to Gygax about it by March of 1971?
Most else of what I'd say here has already been said.
Jon, this is a cop-out

Why can’t you positively state that you support or don’t support my thesis that material from Arneson ended up in the Fantasy Supplement? One sentence would do.