King went to right side bell keys on Zephyr alto/tenor in the late 30s with a linkage, first Super 20s were right side with linkage, late S20s were right side direct with hinge direction change. I believe Super 20 baritones were both bell keys on left till near the end or to the end.
Buescher went to right side bell keys with a linkage (kind of "behind", really) on the 400 alto, tenor, bari in the early 40s or late 30s (I'm not sure when it was introduced).
Martin stayed on the left (direct action) till the bitter end.
Conn as noted went from split (one linkage, one direct) to left side with the 6M and 10M in the mid 30s, and I have seen adverts where they specifically call out improved direct action.
Selmer went from left side direct (Super) to right side direct (hinge direction change) on the Balanced Action (alto, tenor, bari) although they left the soprano as is until the SA80.
SML was right side with linkage from the 40s on, I believe.
I don't know about 1930s and 1940s Buffets.
What Conn did on the 12M is really not a very difficult or severe change - you move one tone hole around and make a linkage for low B just like the one you already have for low Bb. Were they doing this to try to compete on visuals with the Selmer Balanced Action baritone, but they decided that linkages on alto and tenor would slow the action too much, so they forwent the "no interference with clothes" claim in favor of the "fast direct action"? Who knows, at this point.
Point is, the different companies all took different approaches. I suspect the real motivation was the advertising and the real success of the Selmer Balanced Action layout - everyone had to do SOMETHING to spice up their old split-bell-key models and counter the Selmer claims. (And the Selmer BA was a very fine instrument that really started the dominance of Selmer in the US market.) Some moved the keys round to the right with linkages so the horns looked like Selmer layout, but at the cost of linkages which theoretically slow things down, maybe; others went to both on the left so they had the same "direct action" as the Selmer though they didn't look like the Selmer layout. Of the Americans I think the King Super 20 hinge change was about the only one that (eventually) actually changed to the Selmer layout with hinge rods down the middle.