Interesting. Those numbers were assigned sequentially, which implies 117 predates 120, and that 6x6 predates 6x9 in roll film (though the first few were retroactively applied to existing formats when the 100 series numbering was started not long before 1900). Interesting because NONE of the earliest German TLRs had a center window -- the Brilliant had a starter window on the 6x9 track, the Rolleiflex (of course) had the automatic counter start, and there were a couple models with the three-window system on the 6x9 track. Perhaps this is just because they were made to use the Agfa B2 film, which was identical to 120 (including having only a 6x9 track in the earliest days, though B2 apparently had 6x6 by the time my Speedex Jr. was made, around 1948).
Ah, my film size reference (Dead Link Removed) shows 117 had different flange diameter and length than 120 -- might or might not have fit 120 cameras (the length difference is small), depending on the drive arrangement, but certainly wasn't perfectly compatible. So if a 6x6 was intended to use B2/120, it needed the film counter or three-window system, at least prior to WWII.
In fact, I believe it was 620 that introduced multiple framing tracks on a single film; I've seen 620 cameras in 6x9 and 6x6 dating from immediately post-War, if not pre-War, and the Duo 620 half-frame had a single window as well, I think (pre-War half-frame 120 cameras were, AFAIK, always dual window).