There are a few things that are different between the 74 and 75 wiring diagrams. Check your Haynes manual for the 75 diagrams. (Don't have it? Get it
HERE! You need one, and it's cheap.)
The round connector may very well be for the cat/EGR warning lights. There is a module in the passenger footwell (in the foam block) that counts the miles and then turns on the warning light after a certain distance. I think I remember that there was a round plug for that.
--DD