[xcrewchiefx]

I'm pretty sure it's a pipe plug like the two on the oil filter mount.
Not sure why it's there but it doesn't touch anything in the engine & shouldn't affect anything as long as you put it back.

 

[xcrewchiefx]

Same here! 1974 - 83 . Kinda miss those days. Got to fly in Blackhawks a little before I got out. What a bird!