Dart GT side mirrors

Oh, duh, it helps if I read correctly, doesn't it? I thought you were saying the mirror head itself was differently sized.

Actually, it is slightly. The rear part of the head (the last 5/8" or so), from the widest ridge to the glass opening, is at a different taper. The glass itself is slightly smaller in diameter to partially accommodate the smaller opening 4 3/16" vs. 4 5/16".