Sorry, I've been away for awhile. All '61-'64 409s assembled at Tonawanda that were shipped to assembly plants had a suffix code beginning with Q. In '61 it was only a Q. I've sent Bob a couple photos of original block pad stampings which should illustrate the second letter being stamped later and a little out of line from the other characters. I've looked at a LOT of legit '09 blocks and can't remember ever seeing one that did not have the second letter out of line. I think that's good evidence to suggest that the "T0102Q" (for example) part of the stamp was done earlier in the assembly process, then when the final configuration and end application was determined, then the second letter was stamped. Besides changes of cams and pistons in the various short blocks, the final assembly of the engines could also include different brackets which may have been held under water pump bolts, harmonic balancers, exhaust manifolds, intakes, distributors, etc. I just don't know exactly "when" that first stamp was made.
As for the "XX" s, I believe replacement short blocks got that code (or lack of code). I believe if a complete engine assembly was purchased, it was stamped with the normal correct suffix, since it went through the same assembly line right through to the end before it was shipped. (it was a "spec" motor) Since short blocks weren't fully assembled, there was no way to know what the final configuration would be, hence XX.
I've also seen a few '65 409s with the J codes, and the same held true with them. The second letter was a little off. If I saw a "perfect" restored '62 '09 car and the full deck stamp was in line like it was all in one gang holder, then I'd be worried.
I don't know if you can pass that over to MKIV assembly, but it was a Tonawanda assembly practice through '65 on "W"s anyway......
Hope that helps....
Verne