Four speed:
1) 1971.5 through 1976.5 = raised insert on aluminum base
AND
2) 1976.5-1978(?) = slightly raised insert; insert is 100% acrylic(?) with simulated silver edge
(none pictured in this thread so far)
3) 1979-1981 = flush insert; 100% acrylic with simulated silver edge
Three speed:
1) 1970 through 1973(?) = black ball
(not pictured in this thread)
2) 1973(?) through 1978(?) = raised insert on black base
3) 1979-1981 = flush insert; 100% acrylic
(not pictured in this thread)
I believe that the soft vinyl cover does shrink and recede at either the top (so a gap becomes apparent around the insert), or at the bottom (so the base where the nut portion is encased appears to be well below the rest of the knob) -
or both.
The three speed "version 2" appears to have an insert which IS slightly smaller in both diameter and thickness than both styles of four speed inserts.