This exhibit is a mystery as it lacks a Type designation. Radiotron in later RCA production is a trade name not a type.
The base has short pegs suitable for a bayonet holder. The locating pip confirms this. Probably the UV base.
The RCA logo on the dome.
The wide glass tube envelope is 25 mm in diameter, and excluding the UV base pins is 76 mm tall.
Updated July 12, 2014. shape:wide.glass.tube construction:pinch type:triode base:uv pins:4 pin: