Oh, gee. I don't know nuthin' 'bout no beat up, rusted old motorcycles, Miss Scallop.
Anyway, it is clearly English because Japan hadn't lost the war yet. It has TT pipes, so it must have raced at the Isle of Man. The only old timey British bikes I know of that AREN'T a BSA or Triumph are Nortons, Vincents and Matchlesseseses. I'm going with Norton on this for no other reason than I know how to spell it!