A833 synchro question

Dan Brewer said
The wider keys in your pics are correct for the torq-loc sleeve. Notice there are only 3 of the splines that are cut for the keys, line them up with the slots in the drum. That sleeve should be used with an inner drum that looks like this one.

View attachment 1715774684

Thanks, that's what I needed to know. Drum can be seen in the background - it won't work with any other than the torq loc slider.
Why were the drums cut like this (omitting some splines)?