For example, 307 engine showed up in GM product in 1968, but not in Camaros until Jan 1969. 350 engines showed up in Camaros in 1967, but not in some of the other GM products until much later.
In this case, the question is, did the black plastic PS Pump Cap become "standard issue" for the 69 Camaro perhaps earlier than others.
What we have to remember, this cap is an accessory item, not a new engine, or a newly available option. At some point the PS pump manufacturer started using a plastic cap on the pumps they were supplying to GM. The new cap would pretty much be used across all model lines. It was likely cheaper (and it worked better) to make the plastic cap, so that was why it was changed.
Items like that are requisitioned up in large batches.
However the 1969 PS pumps are slightly different then the 67/68 pump - that may have had something to do with it. The rear return outlet was redesigned to accommodate the accessory holes that were now being provided in the cylinder heads.
Maybe with the new pump design, came the new cap? Just a thought for consideration.
Maybe the old caps were used up, before the new caps were used?
Maybe both caps were being install at the same time? Maybe not?