[Barnsnake]

Actually, we were both wrong on the date for 5K19. It's 10/19/1965. They skipped "I" and I can't count.
In either case, it's 1966 model production.

The casting numbers on the heads would certainly help ID the engine, if you're convinced they are original to the block. They should be externally visible between the exhaust ports (near the center as I remember).

As far as an "authoritative" confirmation that Ford didn't stamp vehicle numbers into engines prior to 1968, maybe someone at Ford could help. A statement from a noted engine builder might also suffice. Frankly, I'd think a government agency tasked with checking engine numbers should have figured it out by now. Any chance you're dealing with a newbie?
BTW.... Certain Shelby and special high performance packages were sometimes stamped.