RCL Mark 1 Model 3 End Window GM (late 1940s, early 1950s)
This glass walled end window GM was manufactured sometime around 1950 by Radiation Counter Laboratories, Inc (RCL) of Chicago, Illinois. In the Atomic Energy Commission Radiation Instrument Catalog, it is described as a "fusion seal mica window GM tube for beta ray counting." The catalog also indicates that it came in two versions: one with a window thickness of 3 to 4 mg/cm² and the other with a window thickness of less than 2 mg/cm². The window of the Model three shown below is 1.5 mg/cm².
The photo below shows the cathode, a stainless steel wire screen, used in the Mark 1 Model 3X. Unfortunately, I have no information regarding this version of the Model 3. The X might either stand for experimental or extended. Probably the latter—you wouldn't expect an experimental tube to be assigned a serial number of 186. Using a screen as the cathode would provide a higher gamma ray sensitivity than the aquadagged cathode of the Model 3.
Size: 4" high, 2" maximum diameter
Window: 1 1/8" diameter, mica
Wall: Borosilicate glass
Cathode: Inner surface of glass wall aquadagged
Threshold: 1100 volts
Base: 4 prong
Gas: Organic quenched helium?
AEC Radiation Instrument Catalog, page date November 1, 1949.