Early vs Late Dually Hubcaps

[kdh454]

First off, my truck is a 75 C20 that was converted to a dually using the suspension from a 76 G30.

So, I've been searching for a set of hubcaps for my dually and finally found a complete set of 73-75 caps. Along the way, I bought one NOS 76+ hubcap, because I happen to like the style a little better and I did not know which version I would be able to find.

Well, today I attempted to install the 73-75 caps and they do not fit! The diameter is slightly smaller than the inside of my wheels (wheels are 16"). Just for fun, I dug out the 76+ hubcap and it actually fits. Did GM change the wheels in 76? That is the only logical explanation I can come up with.

The good news is that I have a set of 76+ hubcaps on the way and I do actually like the style better. I just want to know what is different should I decide to sell the 73-75 hubcaps.