Rather then establishing when Chainmail went on sale, I'd say that the April IW establishes when you could have ordered Chainmail if your only way of learning about it was through the IW - there was no IW sent in March. But as I said above, Dave Arneson was not "just anybody," and he had other ways of learning about it.
You seem to be implying that Arneson was able to get the
Chainmail booklet before the general public, for that seems to be what it would take for the statement that Arneson “design[ed] his new game around the fantasy elements of the just-released
Chainmail” [PatW 65] to be true. That doesn’t seem to be supported by the documentary evidence that I know of. You may recall this map and the accompanying letter:
The SoB folks posted it to Facebook back in 2016. I started helping with their research in 2017, and in 2018 when the letter and map came up in an email thread, I identified it as being very old (due to the mention that “Bill Hoyt rules Willamfort,” as Hoyt moved away before Blackmoor took off), and that Blackmoor was already a fantasy campaign (note that Arneson said that “it is partially fiction,” and referenced a “red wizards coven”). Since then, they’ve stated publicly that the letter dates to March 1971 (though I see some evidence that some of the material dates still earlier, so I believe that their March 1971 dating is a conservative one).
This letter and the accompanying map causes more problems with the thesis that Arneson had “design[ed] his new game around the fantasy elements of the just-released
Chainmail,” as even if we assume that the letter and map (and Arneson’s fantasy campaign) dates to March 1971, this would mean that
Chainmail had to have been published still earlier (note that the earliest ad for
Chainmail that
@zenopus found dates to April of 1971)
More importantly, there are absolutely no signs of influence from
Chainmail in the letter. Note that none of the unit names mentioned in the letter are unique to
Chainmail, and while
Chainmail has Wizards (as did many fantasy books at the time),
Chainmail makes no mention of “Red Wizards.” Additionally, although the letter was “submitted for his majesty’s approval,” a likely reference to the “King” of the Castle & Crusade Society Rob Kuntz, it was almost certainly sent to Gygax instead--this is because 1) Kuntz was 15 years old at the time, 2) Gygax and Kuntz had been playing together in Arneson’s Napoleonic simulation campaign for months by this point, with Gygax being the point of contact for their team, and 3) Arneson and Gygax had already been corresponding with each other due to their collaboration on
Don’t Give Up the Ship! If Arneson had “design[ed] his new game around the fantasy elements of the just-released
Chainmail” and sent a letter to the author of the Fantasy Supplement about it, he almost certainly would have at least mentioned
Chainmail or the Fantasy Supplement in the letter—yet he makes no such mention.
The letter has other inconsistencies with the narrative laid out in
Playing at the World that appears to have been based on the thesis that Arneson “design[ed] his new game around the fantasy elements of the just-released
Chainmail”. For example, Arneson states “the area known as Jenkin’s Land is ruled by Sir Jenkins […],” indicating that by March of 1971 Sir Jenkins had already gained his honorific “Sir” title. Yet,
Playing at the World indicates that Sir Jenkins didn’t gain his title until, at the earliest, sometime between May and July:
The next issue of COTT appeared in May, the month that Arneson graduated from college, and promised "the start of the 'Black Moors' battle reports, a series dealing with the perils of living in Medieval Europe (or at least as much as is possible when a wargamer cum fantasy nut creates a parallel world that includes perils from a dozen Fantasy plots plus a few of his own). Immediately afterwards, however, Arneson left for a post-collegiate jaunt in Sweden through July of that year, so it would be some months before the Twin Cities gamers would return to those "Black Moors," or, as the setting came to be known, Blackmoor. […] Once transplanted to the town of Blackmoor, the players led a defense of local soldiers against the invading forces of the vile Egg of Coot, a power that resided to the north of Blackmoor, after the current Baron of Blackmoor, aptly named "The Weasel," defected to the forces of the Egg. Thus, for example, the aforementioned Dave Fant (in the Napoleonic Simulation Campaign, Emperor of Denmark and subsequently Austria), who proved instrumental in the repulsion of the first Coot invasion, became styled as Baron Fant, and assumed control of Blackmoor Castle. Duane Jenkins was styled Sir Jenkins as a result the same action […] [PatW 65-67]
(as a side note, from talking with Arneson’s players, Arneson’s destination was Norway, not Sweden)
Playing at the World also suggests that Gygax’s Great Kingdom was the starting point for Arneson’s Blackmoor:
The leadership of the C&CS began to grant to the nobility certain holdings in the Great Kingdom, all for the purpose of eventually starting a large-scale game of feudal conflict within the Kingdom. […] Though the grand diplomatic game of the Great Kingdom was never fully realized, some members, like Dave Arneson, elaborated their holdings into rich scenarios. [PatW 32-33]
Yet, there are no geographical features in the map that Arneson included with his letter indicating that it was meant to fit into the Great Kingdom map first published in
Domesday Book #9. Keep in mind that it was Arneson that created the stencil for the map of the Great Kingdom that appeared in
Domesday Book #9, though he purportedly based it on a map drawn by Gygax.
If Arneson had drawn the map of Blackmoor included with the letter after he had drawn the
Domesday Book #9 map, then he likely would have made sure that it fit properly into the
Domesday Book #9 map, but it does not—again suggesting that Arneson’s map of Blackmoor predates the Great Kingdom map. Similarly, if the Great Kingdom’s description and map was published in
Domesday Book #9 prior to Arneson writing the letter, he almost certainly would have referred to the “Great Kingdom” rather than the “Empire of Geneva,” which appears to have been a placeholder name conceived by Arneson for what would eventually be called the Great Kingdom. Note that in addition to the Great Kingdom map,
Domesday Book #9 also included an ad “announcing”
Chainmail (see Rob Kuntz's post earlier)—suggesting that
Chainmail hadn’t been published yet. All this is consistent with the analysis at the top of this thread (since it requires that Blackmoor predated
Chainmail), but poses difficulties for the thesis that Arneson “design[ed] his new game around the fantasy elements of the just-released
Chainmail”
(Disclaimer: Although I have contributed research to SoB, I am not under contract, haven’t been compensated in any way, and haven’t seen the final cut of the movie. My opinions are my own.)