340 T/A head identification

OK I just picked up a 340 the block has a casting date 4 8 70 (April 08 1970)? The Block Number is 2780930-340-1
The heads are a little more interesting they have a casting number 3418915 A and on the top side there is a J and on the bottom side that faces the block there is a U.
So yeah they have both a J and a U??
Any ideas? Not sure I may have missed something on this thread but I don't recall anything about two letters on each head.

Reviving an old thread once again!

340 head.jpg

340 head Casting Numbers.jpg

340 head U.jpg

340 head J.jpg