[bcmiller]

The 3857578 M20 tag is not correct for Z16.

It’s rare, used for 65 Chevelle with 3.70 (RPO G75, 96 units) or 3.73 (RPO H05, 293 units) and 1965 Nova with 3.55 (RPO G96, 120 units).

3857577 is a common M20 tag - correct for Z16.

Z16 was originally supposed to get 3.55 or 3.73 but ended up with only 3.31 open carrier. Apparently Posi units were hard to come by in at least part of the 1965 model year. There were 2 designs for Posi in 1965. The first version apparently wasn’t that good. But the later version was used after that from 1965-72.

Thanks to several people (Jeff H. Warren L. and others) for helping me figure this out.