The specimen shown is an ML6C (with ceramic base) of WWII vintage. The ML6 was simply a version of the ML4 adapted for operation in sets powered by vehicle batteries. ML6s were widely used as RF oscillators in medium-sizes military transmitters.
This exhibit has lost integrity in the cement holding the envelope to the ceramic base. Using cloth adhesive tape to repair the valve rather than discard it appears to have been common practice.
The electrodes mounted high in the envelope. Not aimed at more than HF use.
A closer view of the main electrodes. The cathode is rectangular with a flat face that the control grid passes across.
The anode is about 8 mm front to back.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 98 mm tall.