If the main format was 6x6, it looks to me as if the alternate format would have been 645. You could get 6x6 with the window in the center. To do 645 you would insert a mask and then use the numbers for 6x9, first in the left window and then in the right window (i.e., you would shoot with "1" in the far left window, advance the film so that "1" was in the far right window and shoot another frame, etc.).

I'm not sure, though, what that top center window would be for, nor why the windows are numbered the way they are "1,3,5,7" and "2,4,6,8", unless there was some sort of mask for yet another strange format.