The Lanchester Mk. I submachine gun was a British copy of the German Haenel-Schmeisser MP28/II. It saw some service in the early years of World War II, mostly used by Home Guard and Naval units, but most land British forces used either Lend Lease Thompsons or Sten Guns.
The 50 round Lanchester Mk I magazine is interchangeable with the 32 round Sten magazine.