Years of examining and dissecting starters and poring through factory parts manuals.
'62 (first year for the gear-reduction starter) thru '69 has the brush plate attached to the aluminum reduction gearbox/pinion housing with a single philips head screw accessible only after removing the starter field frame. '70-up starters have the brush plate secured to the aluminum gearbox/pinion housing with two screws on "ears" flanking the large and small electrical terminals. With the exception of the '63-all plus '64-'69 170 4-smaller-series-coils starter, the starters are all electrically identical '62 thru early '73: 3 smaller series coils, 1 smaller shunt coil. The change to 4 larger series coils did not occur until three and a half model years after the change to the 2-external-screw brush plate attachment.
All small-type field frames are interchangeable; most "remanufactured" (junk) starters have the 4 larger series coils.
Because the gear reduction ratio is identical for all the small-frame starters, yes, cranking speed and starter torque are directly correlated: more torque = more cranking speed. The large-frame starter has (much) more cranking speed and (much) more torque, and is also quite a bit less noisy assuming the gears haven't been abusively/abrasively cleaned in the "remanufacturing" process. All the large-frame starters (except my one-of-none unit I linked above) have 4 series coils.
