The E2631 is a development valve from M-OV. The construction is as a half wave rectifier with the cathode taken to the top cap. Having the cathode taken to the top cap is indicative of a flyback efficiency diode as used in television and radar CRT displays.
The heater looks to have a 300 mA rating at a voltage of around 20 Volts. This points to use within series heater chains. Consumer electronics was made without mains transformers to save weight and cost. Professional equipment used parallel heater systems with mains transformers for isolation of the chassis. This then looks like a prototype device for the domestic receiver market.
We have two specimens of the E2631 with serial numbers of 25 and 30.
The heater looks to be contained in an inner tube allowing extra heater/cathode insulation.
The anode is blackened for efficient heat radiation and the side fins form an X with ach fin at right angles to its neighbour.
The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 93 mm tall.