The M6 was the standard periscope used on the Sherman tanks. There are two large prisms assembled in each end of a metal shell. Frequent problems with moisture and condensation resulted in the development of a new periscope made from a solid block of plastic with a reflecting surface at each end.Wartime significance.Sherman Tank Periscope Type : M6.Periscope M6. Minneapolis - Honeywell Regulator Company. #96812 1942. D.A.