These were in use up to the 1950's in Uk armed forces , police and hospitals,......the equipement requiring recording camera is set up with this item in place on the microscope, gunsight, or oscilloscope etc., and sharpness and content checked on the ground glass screen,...Then , when all is ready the technician replaces this item with a loaded camera and tells the operator 'OK'.
It would be interesting to know if this has the 'broad arrow' mark or police or medical cipher as most of these are thus marked. The RAF used these during and just after WWII to set up the many oscilloscopes used in aircraft to experiment with early radar. This was considered so important that at the end of the war the remaining parts an dproduction line tooling for the pre-war Leica was removed from Wetzlar and set up in the UK in the hands of 'Reid and Seigrist Ltd' , a key aircraft research company. Although they later sold camers under their 'Reid' brand name, the bulk of their production went to the RAF as oscilloscope cameras including some 'set-up' bodies very like this.