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).