The "outside rear view mirror" listed in the 68 AIM as "3914753 mirror asm." (Take a look on page 46, UPC 1, C2.) is still listed in the 72 P&A, described as "68-69 Camaro, 3914753 L.H., outside rear view (rectangular)"; so I'd say one design, rectangular.
My original 68 mirror is that rectangular design, but maybe others can document a different one...
How is your's different?