The 6C4 triode was specified as both an amplifier and an RF power valve. In the latter role it would deliver 5.5 Watts in Class C. With a very small electrode size in a small envelope volume, this would make it ideal for hand portable equipment.
In larger apparatus it could deliver enough drive power for much bigger valves in Class C. Class C drives the control grid positive and when positive the grid draws current and thus the drive valve has to be able to supply this.
The National Union brand and Made in USA. The anode box is ribbed for strength and the side flanges increase the area for greater heat dissipation as well as providing a fixing point.
The anode seen from the side.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.