Delco (or any supplier, for that matter) made what the Motor Divisions specified.
If Pontiac released two different parts for production, that's what was produced.
In this case, if the metal housing fits the lens and the lens fits the valence panel, you could probably get away with it.
Is there a noticeable difference in the lens/refractor pattern or in the position of the bulb relative to the lens?
Is one of them for the "cornering lamp" option?
I would have guessed that the Bonneville part would be larger and wouldn't fit the Tempest/LeMans/GTO valence panel.
1968 Pontiac GTO
1983 Pontiac Bonneville (G) wagon
2008 Pontiac G8 base