What you're looking for is going to be C5DE-6015-B I believe. C5 = 1965
DE = Falcon (1960-69) Engine [I believe the only mustang casting codes for the engine are V8s, which will be ZE? Someone correct me if I'm wrong there]
The 6015 means that it's an engine part (part of the 06000 series of parts)
The 15 in the 6015 I believe means that it's an engine.
I can't recall what's up with the A or B at the end... I know for V8 A would be a 2v motor and B would be a 4v, but I can't recall the 6 cylinder difference.
Phew! But yeah, go ask him for the first 4 digits of the casting code, should be C5DE if it's from a falcon or C5ZE if it's from a mustang.
Edit: Maybe the B stands for Block? I know some have E at the end for Engine. ??? Maybe...? Haha.