This exhibit is a 1915 Experimental Pliotron from the William White G E Museum Collection, Schenectady, New York. Tyne collection. It was never fitted with a base cap.
Thanks to Joe Gruber for sending the image.
The envelope is mm in diameter and, excluding the base pins, is mm tall.
References: J Gruber & 1047. Type Pliotron was first introduced in 1915.