Ok,.this is different. Sure grip rear??

Ok,.this is weird. I just purchased this 66 Valiant a few days ago and got it up in the air atthe shop to inspect the under carriage. Everything is really clean and looks good. So, I get back to the rear and find a "Use-Sure Grip Lube tag bolted to the cover. I said to myseft,.no way!! Sure enough it's a posi. I never seen this before with a 6 cly car before with 5 on 4 lug pattern. Was this an option or what do I have here? Weird..