Miniature wire ended valves, like the 505AX were developed in WWII for military use, and for hearing aids.
This valve could have found use as the first stage of a hearing aid.
The thin glass tube envelope is an oval tube 9 x 7 mm, and excluding the wire leads is 30 mm tall.
Absolute Maximum Operating Conditions¶
Updated April 26, 2013. shape:thin.glass.tube construction:all.glass age:1950.1960 base: pin: