[6pack] Cylinder Block Identification

[Brian]

Dear Listers:

I would like to submit a mystery for discussion. I'm doing a top-end rebuild
in chase of a leaky intake valve on my 72 TR6 (CC79085).

When I pulled the head, I noticed that it has a recessed-bore block type.
My car was built in Jan 1972, and I thought the recessed-bore blocks
appeared after 1972. My understanding is that my car has the original
engine. The stamping on the deck boss is 7717 U. The block was decked when
the engine was rebuilt a couple of years ago, so perhaps the prefix is
machined off. The numbers 7717 U itself must yield some clues.

When were recessed-bore blocks introduced?

Thank you,

Brian J Alwin
Minnesnowta