Radium Chemical Co. Exposure Calculator (1940s)
A circular (six inch diameter) slide rule distributed by the Radium Chemical Company for use in industrial radiography. Its specific purpose was to calculate the optimal exposure time. Input parameters include the source (radium) activity in milligrams, film type, film to source distance and the thickness of the metal being imaged.
The back side of the calculator is blank (which explains why it is not shown).
As a guess, I would say that it probably dates from the 1940s.
See what a radium industrial radiography source might have looked like.
The cool zippered leather case it came in is shown below.
Size: 6" diameter