For a little added info the MkIII helmet production began in late 1943 and overlapped with MkII production. MkIV production began in September of 1944 so one should be able to find war dated MkIV's for their collection. In David Gordon's book "UNIFORMS of the WWII TOMMY" he details a trials MkIII helmet with an airborne suspension and chin strap in a failed effort to make a 'universal' helmet for all the branches. That would be a highly collectable helmet if one was found.