how far into the model year did left over "X" heads * 2.02"/1.60" (cast #2531894/part #2531902) get used?

how to ID an authentic "U"/"O" head * 2.02"/1.60" (cast #3418915/part #3577053) used for a '70 model year car?
were there other letters used "J"/"Z" in '70?

when did the "360" casting start to show up on #3418915 heads?