Would you know if WHAT's callsign was assigned sequentially? Many early stations have an "A" as the third letter, which usually indicates that the calls were part of a series (if they don't stand for anything that makes logical sense.)
Actually WHAT wasn't a sequentially-assigned callsign... It's unclear (to me, anyway) what exactly it represented-- or if it was even requested at all (random assignments had taken the place of sequential ones by the late 1920s)...
You're absolutely right about the third letters of early callsigns... Vintage sets of call letters with either an "A" or a "B" in the third position dating from 1922 thru 1927 are sequential assignments, at least nine times out of ten... One of my biggest radio geek pet peeves is the re-writing of history; the phony "slogans" folks claim call letters "stood for"... I blame the myriad of Internet pages floating around with bogus "histories" for the bulk of this (Wikipedia pages alone are chock full of errors, but there were many inaccuate ones long pre-dating that resource)... I suppose it's neat and romantic to say "W-whatever" stood for something remarkable, but the truth is most of these supposed meanings were really just backronyms-- if they ever really existed at all.
Some really early callsigns did actually stand for things, even before call letter requests were accepted by the Department of Commerce... Callsigns like "KOP" (Detroit Police Department) and WJR ("Jewitt Radio") came before requests were allowed-- but their grantings do appear to have been intentionally-timed. And no, the "JZ" in WJZ did not stand for "Jerzey". The straws some will grasp at to find a hidden meaning in callsigns are, and truly should be, unbelievable!
Not that I expect anyone to do this, but a trip to Maryland to inspect the original records of the Department of Commerce (and later, it's Federal Radio Commission) will pretty much verify all of this... Now if only we could get people to stop repeating other nonsensical myths of lore like, "Marconi invented the radio", or, "Broadcasting began with KDKA in Pittsburgh"...