I find it hard to believe there isn't a good understanding on the subject - (CORRECT '68-'70 b-body rear view mirrors)
--- & the parts book isn't making a convincing argument.

per the parts book:
2 different mirrors: '68-'69 VS. '70
2 different "bracket & shaft" for convertibles: '68 VS. '69-'70
2 different "bracket & shaft" finishes: convertible "chrome" VS. all else "painted"
3 different "bracket & shaft" for non-convertibles: '68 VS. '69 VS. '70

appears to be:
2 different mirrors: early '68 (no border) VS. late '68-'70 (plastic frame)
2 different brackets (bases): 2 corners/2 rounds (convertible) VS. 4 corners (all else)
3 different shafts: long "Teardrop" VS. short "Round" VS. short "Square I-beam"

I'll get the specifics from my brother's '68 GTX (September '67 built St. Louis car).