I can't help comparing this to owning an unopened bottle of wine of a bygone vintage: once it's opened, it's opened and all the mystique is gone. It might have turned out to be a nice bottle of wine but it might have been better never to know.
I'm asking uk.d-i-y for some scientific advice. Should I presume that the base is celluloid, in which case what are the odds that the box only contains a sticky gloop or crumbled powder? I've heard of ancient movie film stock spontaneously combusting: is there any danger of that and are there any specific precautions I should take?
If the odds are that the film is viable then I'll probably use it - you can get some interesting effects from out-of-date film though the oldest I've used so far only goes back to 1980 and the results I've had with it have been pretty good. If the chances are pretty hopeless I'll probably try and preserve the mystique and keep the box and its secrets intact.