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!
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