Wheel Lug Base Circle, 5 on 5 Only?

[Mean 69]

My wife recently inherited a solid 74 GMC stepside, which we are going to keep in the family and go through (completely). I'll post some shots once we get moving on it, but I was looking at it over the weekend and was puzzled by something. The truck currently has aftermarket wheels, which we are going to replace with something a little "cooler," and I confirmed that the base circle on them is 5 on 4 3/4", NOT 5 on 5" which is what I thought all 1500's came with? This is actually good news as I have a set from a different project that will fit, but begs the question is the truck had been previously modified. I haven't pulled the wheels off to check into it more, thought I'd post the question for the experts here.

Did the light duty GMC's indeed come with the 5 on 4 3/4" base circle, or is there more to the story on this truck?

Thanks!!!
Mark