I had received a check recently and wrote about it too https://news.ycombinator.com/item?id=49286258
I would say he could have even gotten away with “functionally infinite number of alphabets”.
Really need to find another error or point of improvement so as to get the digital account --- I've been reading through TAoCP Vol. 4 Fascicle 7 on _Constraint Satisfaction_, but it's been slow going (though I did find out that it is intentional that he not list beginning of chapter quotes in the index).
I only managed to get one check (for an error and a point of improvement) for _very_ minor points in _Digital Typography_ which was facilitated by my having been fascinated by typography since reading Oscar Ogg's _The 26 Letters_ and his cousin Warren Chappell's _The Living Alphabet_ when I was 8 or so.
(Hmm maybe I should also write a blogpost about this)
That claim goes to the Polish cryptoanalyst Marian Rejewski instead, of course (https://en.wikipedia.org/wiki/Marian_Rejewski); what Alan Turing and Tommy Flowers did was automate the breaking, which was important because it had to be done again every day (due to the many combinations of configurations of the Enigma).