When I acquired my 73 comet, it also came with a gas cap, but doesn't seem to fit, is this the right one?
The 4 ear caps came from later cars. You can swap out the filler pipe. Someone will correct me but I think the change year was 1975
Just to be clear, Maverick fillers can be different from Comet fillers (bolt up different to the car) and there are several variations of the caps. Make sure if swapping you get the right one. Not sure about grinding off 2 of the tabs. You would have to compare a cap that fits to the new one to see if anything lines up the same. The filler is easy to swap. If it were me, I would do that before grinding on the nice cap.
Are fillers easy to find, auto parts stores, or are these rarities that you'd have to get from a parts car?
You will not find them at the store. Just parts cars. But there are plenty around. Hopefully someone will post here what the correct year the change was made. There might be someone that tried grinding off the ears already too. Hopefully someone will post info on that.
Another option would be to buy a two tab Comet gas cap. There are usually some on Ebay and I think there is someone on one of the Maverick/Comet Facebook pages that currently has one for sale. You could then sell the cap that you have.