GM / Delphi sold off the rights many years ago (last model year of use is now 23 years ago), so they are 'aftermarket', no matter what label is on them. There have always been rumors about different grades of them afterwards, but it is hard to say. All I can think of (being the R4), is 'the noisy ones get the AC-Delco labels, the really noisy ones go the the chain stores, and the really noisy ones that leak get Acme labels!
They were all made by Compressor Works at one time (after Delphi stopped), but there is at least one other on the market now, that claims to be a scroll packaged as an R4. It looks noticeably different too, but appears to fit the same. The auto A/C guys debated whether it could really be a scroll and work properly with such a large diameter clutch as packaged, i.e. turn so relatively slowly.