The E2188 is an experimental development valve from M-OV. The envelope is opaque and the inner structure is hard to determine. The pin connections given below have been searched for and nothing matches.
The rod on the right, if it is a connection, is attached above the top mica.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 60 mm tall.
Updated January 06, 2022.