The 938OS is a mystery triode. Clearly a battery low power device with four filament strands passing through the grid.
Thanks to Bob Hopkins for sending the image.
The balloon envelope is mm in diameter, and excluding the B4 base pins is mm tall.
Updated April 27, 2013. shape:balloon construction:pinch type:triode base:b4 pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f