The E351 is an experimental valve from M-OV. It is an upright screened grid valve with an envelope that retains the short tubular section that allows the valve to be mounted through a chassis in the same way that the original S625 was designed to be mounted.
The S215 was the first upright screened grid valve from M-OV and has a 2 Volt battery filament. The next development was the MS4, again with the tubular lower section but fitted with an indirectly heated filament. The E351 looks to be the prototype for the MS4.
The valve says 4.0 V on the glass and when powered takes 1.0 Amp exactly at 4.0 Volts. The base is B5 and the heater is connected to pins 3 and 4. The serial number is No. 54.
The operating conditions presented above are those published for the MS4.
Not much to see except the lettering: Manufactured by The M.O Valve Company Ltd.
Through the glass the anode can just be seen. It is formed as a flat plate backed by a tray with raised sides. Thus it is a shallow box. Presumably there is a second anode on the other side.
With additional lighting to penetrate the gettering the anode is clearer. Also the central screen grid box with the mesh front and back becomes visible. The same screen is used in the MS4 but the anode is replaced with a single plate with ridges pressed into it to confer stiffness.
The classic envelope is 44 mm in diameter, and excluding the B5 base pins is 125 mm tall.