This exhibit is a mystery. The C4 reference is probably a makers code rather than the Type designation. In any event C4 cannot be found in the literature.
The pinch extends almost to the dome of the envelope.
The five base pins.
Within the envelope is a thick wire filament that is coloured a dull grey. No other electrodes are visible due to the gettering.
The thin glass tube envelope is 22 mm in diameter, and excluding the base pins is 51 mm tall.
Updated July 30, 2014.