The Type 717A is known as the 'little doorknob' (for the doorknob see 703A). Intended as a low-noise front-end amplifier in UHF radar receivers, the 717A achieves 4 ma/V despite its small size. Designed by Western Electric, the 717A was made in large numbers by several US manufacturers during WWII. It was a successful valve although it was soon superseded by improved (and cheaper) all-glass types.
The dome carries the maker's name and Type designation.
The miniature pentode sits horizontally on the glass foot.
The pentode with the 11mm diameter mica supports would easily fit into a B7G envelope.
The balloon envelope is 34 mm in diameter and, excluding the IO base pins, is 36 mm tall.