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This exhibit was an ebay find and has no markings. The four pin base is American and designed to fit a bayonet socket. The valve is clearly a directly heated triode. The marks on the envelope are internal.
If it is American in origin it would be reasonable to use their phrase 'whatisitatron' to describe this valve.
The folded bright sheet anode is pressed to have side flanges that both connect to the support rods and also increase the radiating surface area.
The inner marks look like a failed getter. The top connection, above the anode, is the filament top support. Looking into the top of the anode reveals a broken filament and the upper grid wires have turned blue, probably through being overrun.
The grid is held between two supports.
The balloon envelope is 44 mm in diameter, and excluding the base pins is 106 mm tall.
Reference: Observation.

 

Updated April 17, 2013.
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