series 2 or 3 rear end?

[chevybill]

I want to order a posi traction unit of the 70 c 10 half ton 2wd.
In looking at the information-- they always want to know if it is a series 2 or series 3 carrier. What doe this mean. I know I have a 12 bolt rear end with a 3.73 gear in the rear and I know it is the original rear end for the truck.

Wat does the carrier refer to half ton or three quarter ton?

Any thoughts on this appreciated. What brand of posi unit does the board members recommend? I have looked at Eaton and detroit locker are there others? What vendor do you recommend for the best price?

Thanks for all the replies.