Here we have a British made Mullard 6C5GT, not a common valve. The reason type 6C5 disappeared from the lists fairly early is that it was superseded by the 6J5 which had slightly superior ratings and characteristics but was, for all practical purposes, a direct replacement.
Most 6C5s were of the metal-envelope type whereas most 6J5s were 'GT/G' and therefore suitable for a wider range of applications. NB. Type 6SN7GT is two independent 6J5s in one envelope.
On the left is the anode connection and strapped to it is a second grid. This is a fine wound grid much like a screen grid. The 6C5P has this extra grid.
The metal skirt also has the Type designation.
The two grids can be seen by looking at an angle up from the base.
A closer view of the two grids.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 70 mm tall.
References: Datasheet & 1040. Type 6C5GT was first introduced in 1940.