The VT-100A is an American Government procurement code for a modified or selected 807. This exhibit was made by Hytron and has the Type designation 807 etched into the envelope.
What is interesting is that the base cap carries an old UK stores code of 110E/8 and this maps to a selected 807. Also on the cap is the Air Ministry AM and crown. The presumption is that this valve was for servicing a piece of American equipment that was in service with the RAF.
The stores code on the base and the 807 on the glass.
The British Air Ministry logo.
Visually there is no obvious difference to a standard 807 which suggests a selection for performance.
The anode shape is clearly seen with the two halves stitched together.
The classic envelope has a maximum diameter of 51 mm and a length, excluding the UX5 base pins, to the tip of the top cap of 126 mm.