Not only imported here in the war years, but sold throughout the world for many years after they were sold off as obsolete (did you think "milsurp" was something new?). Millions of muskets from dozens of countries were sold to arms dealers who converted many to shotguns for sale in the U.S., Africa, Asia and Latin America.
I have no idea what the IIMLA marking means, but I think it safe to say it is not factory. The number is an assembly number, put on at the original factory to ensure fitted parts were put back together after finishing.
Jim