Factory overbore
I'm not sure about the Letters,, but it was not uncommon to find brgs, lifters, and I believe pistons 1 and 2 thou over/under..
IIRC ma mopar had a shortage of cranks, and had to use some undersize.
As I remember those motors had a "maltese cross" stamped on them..
That was sooo long ago..
I believe you may still be able to get those 1 and 2 under brgs..
hope it helps