I have both handguns, California configuration, one test fired Dec 13 the other Feb 14. Are the lowers (if that's the correct term) identical? Obviously, I can tell by S/N, but when I'm cleaning our handguns there is the possibility of assembling the barrel/recoil spring/slide on the wrong lower assembly.

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M&P22,2x40-his
M&P22,2x9-hers

They are the same

This is not the correct terminology, but we know what you mean. The correct word is frame. Check out this: M&P Parts Identification


One thing to think about though is sear/striker engagement. The sear is mounted in the frame. The striker is mounted in the lower. The contact these two parts make is what mostly determines the trigger feel. By keeping them married (not swapping them) they will get better over time. This will not be as effective if the slides are continually swapped.

OK, ok, it's not a really big deal, but I just wanted to bring it to light.

What I believe you're really looking for is that it's neither unsafe nor will it damage the guns to swap slides inadvertently.

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Freedom isn't free.

Ras, nice diagram you created & posted.

Agree, very nice and will have to save it...