It's also entirely possible that you have one of the first M&P's that smith put out, and at that time, they didn't build them in house like they do now, which could explain why no barrel markings.

Although I have no idea if while they had the uppers built else where, that they were or weren't stamped as they are now.

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Rod
M&P 22A, 40c, 15A, 15-22