The CV797 is a xenon filled tetrode thyratron. The prototype was the commercial 2D21.
The rectangular box is grid two. The anode connection is in a ceramic tube from the base pin up and into the box.
The STC logo and commercial Type code 2D21 indicating equivalence.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 43 mm tall.
Reference: Data-sheet. Type CV797 was first introduced in 1944.