Another engine ID mystery.

Sounds much like the original engine that was in my 85 D150. It didn't have the stamping on the deck, and I can guarantee that engine has never been apart since manufactured.
I had difficulty finding out the same info on it as the op is looking for.
I had heard that production of the /6 shifted to mexico in the later years and some of the well known ways to ID a /6 went out the window at that point....