This exhibit is another 'Short and Dull' Electron Relay from Atlantic and Pacific Co.
The glass carries the letters ER and what looks like the word experimental. On the base cap are the Marconi and DeForest logos. The DeForest logo can be seen on the right in this image.
Thanks to Joe Gruber for sending the images.
The balloon envelope is 44 mm in diameter and, excluding the UV base pins, is 104 mm tall.
Reference: Joe Gruber.
Updated February 08, 2021.