The 6C21D is a translation from the Cyrillic letters on the case. It is probably a UHF pencil triode but the outer case could be a resonant cavity.
The two wires are probably the heater or filament. The tab connects to the case.
The opposite end has an insulated connection.
The adjustment screw on the case has a wire under the locking washer. The overall length from tap to tap is 70 mm.
The thin metal tube envelope is 22 mm in diameter and, excluding the base pins, is 43 mm tall.
Updated August 28, 2020. shape:thin.metal.tube type:triode base: pin: