The RF30 is an indirectly heated half wave rectifier with a perforated sheet anode and a heater that is connected top and bottom suggesting a single length or coil of wire.
The gettering is to one side only.
The anode forms the main part of the image but note the ceramic insulation at the base of the cathode tube. The rod in the foreground is the connection for the top of the filament.
The classic envelope is 45 mm in diameter, and excluding the B5 base pins is 116 mm tall.
Updated April 17, 2013. shape:classic construction:pinch type:rectifier.hw age:1930.1940 base:b5 pins:5 pin: