'65 273 engine

OK - had a minute to go look.
On the block, under the Driver's head is stamped "A-273 4-24"
Casting date is 3-8-65.

Hopr that helps - I've never been much of a numbers guy but the casting date tells me it was a late production assembly/car. Assembled in Aprill-ish, and in a car before the June shutdown.