Firebird Rear end indentification

[city_ofthe_south]

ok so there are supposed to be two letters on the rear end to ID it with? ie - XB or ZB etc. It's a 68 Firebird and we have no background on the car so we're going on the assumption that it could be anything. However the date code on the diff says it's a 68. one other forum i read today someone was asking this same question and they sited the exact same casting code or whatever it is (9793019) ... anyway, someone said something to the effect of 'that's a Firebird 3.07 rear' ... well they made a 3.08 but not 3.07. maybe he just typed the wrong thing. The guy who started the thread asked 'how do you know that' but mystery man never replied. So back to the two letter thing .... i only see one B ... there is only an XB and ZB i think for 68 ... standard diff and saf T track or however you spell it. I think that the "standard model" firebird came with a 2.56 open rear and we do know that this was a standard model Firebird. However, I can't say FOR SURE that it's a 2.56 or that Standard models ABSOLUTELY came with the 2.56 only. My brother had it open and refilled it and doesn't want to open it again. I'm wondering what's up with the B only and why can't i find any documentation that talks about 9793019 other than one forum ... not even in the shop manual for the car. It's a 10 bolt btw. Another odd thing ... everyone who looked at the rear before I ever saw the car said it's posi (whoa! that's throwing a wrench in the problem eh?) ... just in passing they spun the hub and everyone points and says 'posi' ... you know, like that. I don't think anyone took a close look. My brother swears up and down both sides turned together the same direction .... yeah, they don't do that now, they turn like an open diff. I've tried it a million times. Could something be wrong? could it be a locking diff? i just don't know quite enough about it to get any further than this. thanks.

[PonchoJohn]

"Posi" is short for "Posi Tractraction", Chevy's name for a limited slip differential. Safe-T-Trac was Pontiac's name, Mopar used Sure-Grip, etc. They all mean limited slippage when a tire loses traction.
For a 68 bird, there were several gears from 2.XX all the way to 4.XX gears. Non-Posi: The easiest way to find the ratio is to jack up the rear end so that both tires are off the ground, then make a mark on the driveshaft yoke flange. Now make a mark on the tire. Spin the driveshaft over 2x and count the number of tire rotations. Divide the tire rotations by two for your gear ratio. More than likely, you're going to come up w/ 2.8 rev's- 2.78:1.
Posi: jack up rear, make marks, spin the rear one revolution, the number of turns is your gear ratio.