This exhibit is a 1917 experimental Pliotron from the Gerald Tyne Collection. This has a UV base cap with brass skirt. Note the locating pin for the bayonet base.
The conical anode Pliotrons were made for low power receiver applications.
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: