That sounds reasonable. Has anyone ever seen some kind of rotating back negative holder that has masks? Could the photographers have done these vignettes on the same neg using a negative holder that had some kind of masking ability? As I think about it sounds pretty complex and I bet as you suggest, the photographer(s) were just taping cardboard with the masks cut out to get multiple non overlapping exposures per one negative
Revolving back would have left one image upside down.
Rotating back would have left one image inverted (i.e. 'round the wrong way/back to front).