Merlins were always despatched complete.
The large sub-assemblies such as superchargers were generally machined and assembled in-house, which would also include Ford at Trafford Park, and Packard in Detroit.
A huge number of outside contractors, including many car manufacturers, were involved in the overhaul of aero-engines, and this often gets confused with manufacturing.
It is not impossible that used Merlin superchargers were being used for something else. Post-War, the CEGB used Griffon superchargers for something or other.
The Rolls-Royce Heritage Trust could give you a more definitive answer regarding your particular site though.