250 CID head

[Zogbert]

I've got a 1972 C-10 with a L-6 250, casting 3994257. I looked at the plugs, and they aren't tapered, which the stock engine should have had (R-46T). Upon inspection of the plug hole, the hole has no taper. The head is a greenish color, while the rest of the motor is orange. It's obviously been changed.

The plugs in there are AC Delco R44XLS, which is the correct plug for a 1966 to 1969 250 CID and older 230 CID engines.

Any way for me to tell what year and engine the head is from? I don't see any casting number.