My 75 coupe did not have a front air dam when I bought it a few years ago.
Questions:
Was the air dam standard on the 75's?
Does the air dam help significantly with cooling?
Yes.....and Yes. They also tend to get damaged on angled driveways, or parking lot curbs....which might be why yours was/is missing.
Yes and yes. I assume you are talking about the 2" black plastic lip underneath the front end and in back of the large openings. Probably got damaged by a previous owner and removed.
1973 L-82 4 spd