Atlas, Stant and NDH all supplied the different standard caps over the years, they also had their own locking caps. The GM locking caps just look like these also, in a GM box. You can still buy them for that traditional look, the brand name items being the same, usually minus the stamped GM numbers, same as the regular caps they also still sell, or did. Also note, I've never seen any that feature GM numbers, usually chrome top, perhaps something to ID on inside.
I picked up one cheap, just because. By cheap, meaning it was about 100 less because it was Atlas brand and not in a GM box, but meh....
Of course, any of the more recent black plastic caps that fit the 80's and 90's GM cars will usually also fit. Their locking version often is the same profile but lock barrel in raised handle, so this type may or may not fit under all license plates without touching. Often, the many differences are in the pressure valves for different emissions setups which were still fairly basic when first seen on the original vented caps.
I just dug out a couple of Accessories Brochures and parts manuals and couldn't find any fuel caps, but they are there somewhere. Perhaps in the large black dealer folders I have stored away. I don't think it had part numbers, just the typical period drawings in between luggage racks and dash compass (70's GPS).
But that's all rambling, you can still get the aftermarket types at a fraction of the price. You may find that having to swing keys about unlocking it each time more trouble than it's worth. Mine is still in box somewhere.
