The first i ever saw were a odd two piece sabot from T/C that also has a felt wad on the bottom. They came in tubes with the bullets/sabots "assembled". IIRC they used a 240 or 250gr XTP.

T/C Break O Way sabots


The rest of your answer to the question is probably correct. Subs and sabot design have improved a bit since then and now we have many more options. Back then though, even a 209 primer in a ML was rare and slower twists were very common vs today's inlines. Lower velocities and slower twist made it nearly impossible for a long bullet to stabilize. Pistol bullets were/are usually shorter and the fps was a fair match for magnum pistol speeds.

This is a pretty good link to some the history.
http://www.chuckhawks.com/muzzleloading_bullets.htm