Did you have to order the posi or did it always come standard on the z"s

With the standard 3.73:1 axle [or numerically lower] positraction was neither included nor a mandatory option but only on the Z/28. For any other 1969 Camaro with 3.73 or numerically higher axle positration was mandatory.

That makes a "BO" code 3.73 12 bolt open axle unique to a Z/28. My last Z/28 had one but was rebuilt with positraction. 4.10s and up were all positraction.

Great info. Does the code show up on the rearend housing?

Yes. it does. Both my BB cars have a "BS" stamping on the pass side of the axle tube which is 3:31 Posi.
FYI -- Most rear ends have a cast date of approximately 2 months prior to the cars build date. At least this is what I have noticed with all my cars past and present. My convertible's body's build date is 03D and the rear end is a A-31 if I remember correctly. Although my friend had a COPO and one of the ZL1's that both rear ends were produced 4-6 months prior to the body's build date. I hope this hopes.

Yes, "BO" on the RH axle tube.

Mine has BO. Which one is this? Is it still the Z28 rearend. It is a 3 series carrier that is not a posi.

Im sorry. The letters are BL

BL looks to be a 3:07 non posi. My car is a real z. Could this be the original rearend.

My car rolled off the assembly line with the mighty BO coded rear end. I currently have 3:55's that came in the car when I bought it. I need to get the originals back in sometime soon. The 3:55's stink for around town.
Doug.