[71 super]

PT look at the spindle arm where the drag link attaches. There should be an aluminum band that looks similar to a early zip tie attached to the spindle. This should provide all the info. on the spindle- ie the build date and possibly the code as well.
As far as the option list, that is a GM list, but it does have 10-30 series and 40- 60 series as well as el camino options included. That is the truck master list. It is possible that it is for the camino but the have the car pattern and still have the 7/16 studs I believe. Another item, these trucks share alot of parts with camaros, chevelles and the big car line- mostly trim items, do a little comparison to the back up lights and cargo lights on 68 camaro- same stuff exactly. Also look at the gas trim and the clutch pedal trim from same cars it is the same. In saying this I also believe that it is possible that it could be an option in 70 simply because the disc technology was already in place but limited to high end cars but very rare. 71 was the first year it was standard and non option to get it on light trucks, when did it become become standard on cars? I could be like seat belts and other options early, rare and become standard either to save money or avoid law suits, just like the throttle rods and cables. Breaking motor mounts ushered in the change for cables and in 72 bb trucks the shift occurred early for motor mounts. My point is it is possible and feel if the tin tag decodes the correct dates coupled with the GM book paul could be correct.