Itinerant Air-Cooled

My license plate light lens (the clear plastic part that fits over the light itself) has finally died. In looking for replacements, I see ones for pre-1971 buses, but not the '71 itself. However, there are also gaskets available (which I'd also need) and these gaskets are for 58-71 buses. So why is the gasket the same but the lens different?

And if anybody can steer me to a source for a da kine lens and gasket for a '71, I'm all ears.