Rear end question

[Nachos82]

I pulled the cover off the rear to change the oil for a trip I am going on soon. I was wondering since it is off, if someone that knows rears can help make a positive ID on this rear. It is in my '67 1/2 ton, the code list in the glove box said it should have the G80 rear in it. From what I have found, this is supposed to be a "locking-differential" rear end. I was just wondering if someone could tell me 100% what I got. I want to try and find the codes on the ring gear while I am in there to see what ratio it is also. Here is a picture for your viewing pleasure, thanks.

Edit: Almost forgot the main reason I posted. Should I be running any type of additive from GM? I know, or have been told you should run some friction modifier with the limited slips. Does this one need it, or is straight gear oil fine for this? Thanks again.