Thanks to Rick Gonser and Len Sokol for providing these documents.
These documents list accessories for 1956. They contain part numbers, dealer price, and suggested retail price for accessories that could be ordered by the dealer. They were meant for use only by Pontiac dealers and were not released to the public. Three of these documents were published through the 1956 model year with each release providing updates to the previous version. They give some insight to when options were released. For example, the 285 hp dual 4-barrel option doesn't show up until P56-3 so that option was not available until sometime after P56-2 (February).
Some 1953-55 accessories are also listed.
1956 Pontiac Package Accessory Price List, P56-1, October 1955
1956 Pontiac Package Accessory Price List, P56-2, February 1956
1956 Pontiac Package Accessory Price List, P56-3, June 1956
I got this document from Len Sokol, it appears to be a factory-installed accessory price list that factory charges to dealers.
1956 Pontiac Factory Installed Price List, F56-3, June 1956