This I had not known, but apparently it is old news:
The symbolic kingpin of interchangeable parts production fell in 1960 when Robert S. Woodbury published his essay “The Legend of Eli Whitney and Interchangeable Parts”…Woodbury convincingly argued that the parts of Whitney’s guns were not in fact constructed with interchangeable parts…
With Eli Whitney reinterpreted as a promoter rather than as a pioneer of machine-made interchangeable parts manufacture, it remained for Merritt Roe Smith to identify conclusively the personnel and the circumstances of this fundamental step in the development of mass production. Smith demonstrated that the United States Ordnance Department was the prime mover in bringing about machine-made interchangeable parts production of small arms. The national armory at Springfield, Massachusetts, played a major role in this process, especially in its efforts to coordinate its operations with those of the Harpers Ferry Armory and John Hall’s experimental rifle factory, also at Harpers Ferry.
That is from David A. Hounshell’s excellent From the American System to Mass Production, 1800-1932. Here is a related article, possibly gated, here is another.