In 1969, Pontiac made a running production line change to the Firebird gas tank, and they went from non-vented to a vented cap. If you have the deep collar cap, you have the non-vented setup, which would be the system where the vent pipe runs from the filler neck up through the trunk pan, and back out.
The short collared cap is for the vented gas tank, which they began using some time after January 1969.
