'72 didn't have the holes in the header panel and grill for the 5mph bumpers compared to '73 and '74.
'72 didn't have the holes in the bumper notches in the fenders and quarter panels for the filler panels too.
'72 had the same rear filler panel as the '70 and '71 challengers on the tail light panel, '73 and '74 didn't have it because of the 5mph bumpers.
'74 got shoulder belts integrated with the lap belts.
'74 got the dreaded seat belt interlock thingy.
'70 and '71 trunk lids were different than the '72 - '74. I think it had something to do with the no wing being available those years.
'72 - '74 gauge bezel was different because of the seat belt light.
consoles were a little different even though they looked the same.
washer bottles were different for '71 compared to '72 - '74, that's because of the charcoal canister.
I think the horns were mounted on the right inner fender in '71, hood latch support in '72 - '74
'72 - '74 got the front valence trim where '71 didn't have it.
fender turn signals, early '72's got the '71 style, late '72's and '73 - '74 got the screw through the top style.