71 and 72 mirrors use the smaller 3"ish mounting.
73 and 74 Mirrors use the longer 5"ish mounting.
Both sides/both styles used a mounting plate inside the door to mount the mirrors to.
Sounds like your passenger mirror is a 73/4 and you are missing the mounting plate for the pass. side.
My SPD 201 Charger R/T has factory well nuts on the left(drivers) door to mount the side mirror. The original mirror was a non-racing style which are not as heavy as the racing mirrors. This made me wonder if the backing plates were used for extra support for the heavier racing side mirrors? But I've never run across any indication of that in print anywhere.
