mine was so eaten through on my '74 that parts were just gone... the upper left corner was missing around the tail light lol.
i ended up getting a '70-'71 tail panel from AMD it's on the car now. with that tail panel you also need the left and right mount for the housings.
i got two sets of the housings (one from ebay, one from here) and between the 4 lenses that came with, i had 2 in really good shape (needed only minor repairs in the plastic.)
the '74 tail panels are out of production. at the AMD dealer booth at carlisle there was 1 there but they wouldnt let me buy it, since they had promised it to someone ahead of the show.
so you can switch to '70-'71, or recreate yours, or get a '74 from one of the texas/new mexico /arizona salvage places
(note: the '70-'71 setup has an aluminum housing with a red lens that sets into it. the housing assembly then bolts to the tail panel. a frame part on the left and right is in place for the housing to bolt to. not sure on '72,73. the '74 was just a lens assembly that bolts to the tail panel.)
i'd check with AMD before ordering that '73 panel from summit. when i called AMD they only had the '70-'71 available.