I've heard the same as Fastbolt. FWIW the Houlton plant makes both the Model 22A and the Model 41, but I'm not sure about the M&P 15-22 pistol.
AFAIK the 3rd-gen TDA pistol production line only runs for LE special orders now; the reason it was moved to Houlton was supposedly because S&W management didn't want the busy Springfield plant occupied by machinery that's not in use most of the time.
Also, "licensed Walthers" means the PPK and PPK/S lineup, as stated by Pinky. The P22, PK380, PPQ, and P99 are imported from Germany and distributed by S&W. The Walther-derived USA-made SW99 has been discontinued in favor of the M&P series.