Unknown facts about Nikon F90X
F90X will not fire shutter with open back unless it is set for Manual mode and shutter speed of 1/250. However it can be reprogrammed to ignore open back sensor and fire as if back would be closed.
There is an internal limit for number of frames on a single roll of film. F90X will not make more then 99 shots on one film roll. That sets the limit for bulck loaded back if somebody makes one.
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Leo Bodnar, June 2001