It has nothing to do with being "period correct", it's simply personal preference.
Mag extension sticking past the barrel is visually unappealing to me. I prefer them either perfectly flush, or the mag extension at most a half inch shorter than the barrel.
Mag extension not being perfectly parrallel to the bore drives me absolutely crazy, yet several mag extension and clamp combos end up this way.
This is how I prefer my mag extensions to look:
View attachment 917073
IIRC this is a Choate extension. Not my first choice at all - I'd have much preferred to keep the S&J - but this combo ended up looking the best (to my eyes).