I've always assumed he got it the same way Rubinstein, Vidmar, Saemisch, Duras, et al got their GM titles, i.e. by just being awarded it for performances before 1950.

< ... His International Master’s title, awarded by FIDE in 1965, was as much a testimony to his years of pestering the officials involved as it was to his long-gone chess strength.

And yet especially during the roughly 20 years between 1910 and 1930, when not in prison, Whitaker excelled at playing chess. What he might have accomplished in the game, had he spent his time studying and playing it, instead of hatching one failed criminal plan and contentious lawsuit after another, can never be known. To borrow a few lines written regarding his one-time co-conspirator, Gaston B. Means, but that might equally apply to Whitaker, “Few men attain so much fame and so little success in so many kinds of crime, and at the same time acquire reputation of a sort in fields unrelated to their strict criminal activity.” (Quoted from the Christian Century in the Philadelphia Daily News, 28 September 1963, page 14.)’>

https://www.chesshistory.com/winter...

I left the follow-up paragraph in for color and that great quote.

Speaking of color(*), Sam Sloan has this take on the matter:

<Whitaker would travel to Germany every Summer. He would buy a Volkswagen Beetle there, bring it back to the US., drive it around for one year until the next summer, and then sell it at a profit.

I believe that Whitaker played in many tournaments in Europe, but I have never seen mention of any of them. He did get the FIDE International Master title in 1965 and, if he did it legitimately (a questionable assumption), it was from his European results.>

http://www.anusha.com/whitmore.htm

It has a picture of Whitaker and a young Sloan.

* * * * *

(*) Have a look at Sam Sloan's photograph on <CG>. I asked <Daniel> if we couldn't get a better photo of him, but <Daniel> insisted we keep it as it's the one Sloan himself submitted!

<After more than ten years of campaigning, Whitaker was finally awarded the International Master title by FIDE in 1965, based on his several strong tournament results from decades earlier.>

WHITAKER: <Recently, the qualification requirements were changed for FIDE title awards. It is now easier to earn international titles than it was years ago. I was awarded the title of International Master in 1965, under the old qualification rules.>

WHITAKER: <Often I am asked about Bobby Fischer. I know him better than most others. We spent three weeks on a chess tour through many southern states and Cuba. The team of eight voted that I play Board 1, Fischer on Board 2. Naturally I had stronger opposition, but in the end our scores were the same--each won five, lost one and drew one. I urged Fischer to play 1. P·K4. In the dozen years since, as I recall, Bobby has never played any other opening move!>

<Since Taylor Kingston does not know much about chess, he does not know that by 1960 Fritz Saemisch was completely insane. Saemisch would get invited to international tournaments where in every game he would play only about ten moves and then sit transfixed for two hours until he lost on time. Naturally, since Saemisch had the grandmaster title, organizers would invite him so as to manufacture titles for others. Obviously, Whitaker would contact Saemisch to get the international title for himself. Whether Whitaker actually played Saemisch I do not know, although Whitaker told me that he was trying to arrange a match with Saemisch. One question left unanswered by Hilbert's book is exactly how and when did Whitaker get the International Master title. I have asked many FIDE officials this question, and nobody seems to know.>

I'm fairly sure Whitaker did play a match against Saemisch at some point, post-release.

Remember - the ELO system wasn't in place in 1965, so the "norms" were radically different from our modern norms - i.e. they could be rather subjective as to what qualified.

http://www.anusha.com/whitmore.htm

Which suggests a detailed explanation of exactly how the title was awarded is not likely anytime, soon or otherwise.

My personal view is that if Hilbert didn't give the details then I'm not betting on them being found either.

(Hilbert wrote the <Shady Side> biography which I think is the most authoritative source on Whitaker out there)