Rebuilding my doors I encountered a couple of notable elements.
One. There is a tab that is top and forward most of all doors,
very soon EVERYONE should remove these and address the corrosion, or replace!!!

The other is the back mount connected to the (polished) upright appears to be
one of two versions. One version seems to have ...more support.

Is this a early vs late production change?

Early windows have little wheels/rollers in the support to guide the window up.

No, Pics will help.
It may be that there is a an Early Mid an Late version of this window guide support.

I know what you mean. My '76 has small, triangular supports that bolt to the upright with 2 M6 hex head screws. I bought an earlier upright on eBay and it came with a support that also has a tab that goes up through the rubber and directly supports the glass of the triangular "vent" window. I also noticed that NOS rubber has a slot for the tab, but it's filled with thin rubber that can be cut our or not, depending on which support you have.

Looks like a manufacturing engineer was cost reducing it, because the later version is the same left and right, while the early ones have separate left and right parts.

I'll try and get a picture later tonight.

Jim

If I'm reading this correctly, you're talking about the "nub" that fits on the top of the quarter window. If I'm wrong, disregard me as a fool (which many do anyway). This "nub" seats with the top rubber.

There was a difference & significant(question #2). The '75 on 914's had a completely different design. It was, quite frankly, a cheap-out. But, I don't think it can be retrofitted.
Pat