The E800 is an indirectly heated half wave rectifier with the anode taken to the top cap.
The etched lettering indicates that the heater is 4.0 Volts and that the serial number is 56.
The the front to back dimension is 4 mm.
The blacken anode is ribbed for increased rigidity. The cathode tube is rectangular and folded heater passes through the cathode four times. The lettering on the glass identifies the maker as MO Valve.
A better view of the lettering.
The classic envelope is 45 mm in diameter, and excluding the B4 base pins is 107 mm tall.
Updated April 06, 2019.