As said, you may not swap inserts between SL66 and SL66 E camera backs, because the inserts are matched to the backs (to get the film feeler work correctly).
Originally Posted by Slixtiesix
You can freely swap inserts between SL66 SE backs, because these do not have the feeler mechanism anyway. Those are the ones with the crank (so the difference is pretty essential, and not just cosmetical). It remains true that the backs can be exchanged and yes, the meter will not work if there is no ISO dial on the back.