My experience with P90 and P345 magazines mirrors stchman's "The mags are completely interchangeable".
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I only run the mags with plastic followers in my P90 because the metal follower can gouge the the P90 frame.
I've not seen this happen with the plastic P345 frame, but I mostly run the plastic follower with it as well.
Dry firing a P345 without a magazine in place results in the firing pin colliding with the firing pin block. This could cause damage to either/both of the parts. Enough damage can occur to prevent the firing pin from setting off a chambered round.
I'd say it would be prudent that any P345 purchased these days should have the firing pin block and firing pin inspected.
Regards,
Greg