This exhibit from 1917 is a further refined experimental Type G Pliotron that led to the CG 886, VT-1 & VT-11. This has a UV base cap with brass skirt. Note the locating pin for the bayonet base.
The tungsten filament was formed into a helix.
From the Tyne Collection.
Thanks to Joe Gruber for sending the image.
The balloon envelope is mm in diameter and, excluding the UV base pins, is mm tall.
References: J Gruber & 1047. Type Pliotron was first introduced in 1915.
Updated October 20, 2019. shape:balloon construction:pinch type:triode age:pre.1920 base:uv pins:4 pin: