We were told in armorer classes that as the AR15 rifle and M&P pistol production were ramping up, to make room in Springfield they shipped out the 3rd gen production and repair to Houlton. To be fair, they also said they'd been providing new CNC machines and new training for the plant workers. That meant the Houlton plant was making 'cuffs, Walther PP/PPK's, S&W .22's, SW1911's and the 3rd gen guns.

I'd not be surprised if there was some "overlap" in production as the shift was being implemented, but I never asked about it.

Since the 3rd gen guns had benefited from engineering and repair dept involvement during the AIP days (Automatic Improvement Program), it would make sense that there was a lot of experienced, institutional knowledge about the constantly refined 3rd gens located in Springfield ... right up until much of that experience and knowledge was retiring and walking out the door. Of the factory folks with whom I grew acquainted over the years of being a S&W trained armorer, a couple have passed and several have retired. The passing of a generation.

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Ret LE Firearms inst & armorer