Identifying the series of GM 12 bolt rear end...?

[Chrisw]

I've searched and tried to watch some youtube but am still trying to figure out how to identify what series of 12 bolt rear end I'm looking at. Found one for a decent deal somewhat local but... guy thinks is out of a GTO (which if I understand correct makes it a series 3) but also thinks it has 4.55 gearing (which would have to be a series 4 right)? How do guys actually know what series they are looking at.

I've got a 69 Beaumont with a 454 so I'm thinking I want 3.73 gearing (has to be a series 3 right?)...

So confused. Ha!