The rearend in my 70 Malibu is not the stock one. I have no idea where it came from as my car has been idle for 27 years, and the high school hot rod days were such a fog. I do remember the original one was a one wheel squeel unit.
Anyway the one in It now is a 12 bolt ring gear and 12 bolt cover posi.
To my surprise it appears to be a 355 ratio by turning the drum and counting the number of yolk turns method. I Don't think it's ever been apart though and not sure if GM used a 355 ratio??
Anyway it has some springs between the spider gears and looks like some clutch discs on each side .
Is this called a limited slip rearend and do I need that additive that you add to the oil to keep the clutches from sticking and snapping loose?

Thanks for helping id this rearend,
Roger

That would be the Eaton posi, and yes, you need the additive. Chevy did use a 3.55 in these rear ends.