Imperial Lugers are those manufactured between 1898 and 1918 by DWM and Erfurt (the jury is still out on Spandau and probably always will be). The Swiss, Dutch, Russian, Bulgarian, Brazilian, Bolivian, Portuguese and U.S. Test Lugers all qualify as they were made by DWM during this period. I haven't included the "exotics" like Argentina, Serbia, etc. since there is some question about their authenticity.
There is a real good reason...DWM and Erfurt were the
original manufacturers...anything after 1918 is a reproduction.
The first post Imperial Luger was manufactured by BKIW.