This looks at first sight like a Type 3V4, or possibly a Type 3S4 (or direct equivalents). Both types are 1.4V/2.8V output pentodes for battery portables. The two types suggested are essentially similar but differ in pin connections. It should therefore be possible to distinguish visually between them. Both types have 50/100 mA filament rating but it is just possible that this exhibit has a 25/50 mA rating (eg. DL96).
It is impossible to identify the type designation of this pentode valve as all of the lettering has been rubbed off over the years. Rather than discard the valve we took a close look under the glass.
The first feature of notice is two fine wires held in supports at the top of the valve. This is an inverted V filament. The first grid close into the filament is wound on notched support rods and is a fine, slightly flattened, helix. The second grid is wound between notched rod supports but has a more distinct slope to the wires across the face of the first grid. A third grid is to be seen some distance away from the other two, just inside the anode tube. The wire forms a widely spaced flattened helix. The anode is a tube made from thin sheet metal.
Thus it is clear that this is a low power battery pentode. Probably with a 1.4 Volt filament as 2 Volt filaments belong to an earlier age.
The connections from the electrodes to the base pins are clear and could be traced with little difficulty.
The thin glass tube envelope is 17 mm in diameter and excluding the B7G base pins, is 45 mm tall.
Updated March 01, 2013. shape:thin.glass.tube construction:all.glass type:pentode base:b7g pins:7 pin: