68's have the style that have two small (about 1/8" in diameter) drain holes on the bottom of the mirror housing, one on each side (see the top picture). Starting in late 68 (I'm not exactly sure of the exact date) Donnely changed the housing. It now had a single "oblong" 1/2" slot on the bottom, center of the mirror (see lower picture). I've verified this on two seperate mirrors, both dated 7DMI8. One had the "holes" and the other had the "slot"! So sometime in July 68, the change was made. As this is VERY late for 68 cars, I can't be sure if ANY 68's came with the newer ("slot") style, but I would imagine some very early 69's came with the older ("Two hole") style. Also note that 68's have an "Intermediate" Adjusting Ring. Since this same mirror was used on all Chevrolet body styles, the assembly could be adjusted for different mountings. Loosen the screw, adjust the ring and tighten the screw. In 69, Chevrolet went to different mirrors for different models, therefore no adjusting ring was necessary.
1968 (All Body Styles) Remote Mirror with the (2) 1/8" Drain Holes and the Intermediate Adjusting Ring:
1969 Camaro only Remote Mirror with the 1/2" oblong slot and no Intermediate Adjusting Ring: