The STC V237C/2k is a Heil Tube microwave oscillator. The shape and construction confirm this as does the Type designation. However, we can find no details about this precise valve. The V237C/1K is listed (pdf here) but not this one.
The operating frequency range is probably similar to the /1K at 3.6 - 4.4 GHz.
The side evacuation pip. The output is taken from the top cap directly into a waveguide or co-axial line.
The thin glass tube envelope is 19 mm in diameter and, excluding the B7G base pins, is 58 mm tall.
Updated June 20, 2021.