Purchased a 1989 Firebird Formula 350 WS6 W/ out RPO label. The tag on the differential cover says " SPIN RESISTANT DIFFERENTIAL. USE PROPER LUBRICANT". Upon inspection, it is not a "Gov-Bomb" or a conventional Auburn type posi, but something like I've never seen. I would have to say it looks remotely like the Auburn cone clutch type, without the cones. Does anyone know what its called, and are they durable, good units?
Thanks.

 

If it's a stock 10-bolt, it's an Auburn, and a total pile of crap. But if it still works, better than no posi at all.

If it's a 9-bolt, then it's a .... 9-bolt. That uses cones but doesn't need additive because the cones are metal.

RPO labels won't necessarily tell you what it is; been a long time since 1989. Lots of parts have been changed over the years in lots of cars. Better by for to abandon that, and concentrate on THE PARTS and what they REALLY ARE right NOW, rather than "codes" that only tell what they WERE .... near 25 years ago now.

Post up a pic that shows what's inside the "window" and we'll tell you what it is.