The 5C/450A is a pentode transmitting valve with the suppressor grid brought out to a top cap so that it can be used to modulate the transmission. Essentially the 5C/450A is a 'big brother to the well-known type P15 and covers a similar frequency range but at much higher power.
The lower ceramic insulator, anode and suppressor grid.
The four large base pins. The outer ones are the filament, the pin at the top is the screen grid and the centre pin on the lower row is the control grid.
The dome has a glass tube within that is used to support the top of the electrodes. the anode connection is on the right.
The filament support springs with the tape connection to the anode in the foreground.
The wide glass tube envelope is 100 mm in diameter, and excluding the base pins is 287 mm tall.
Reference: Datasheet. Type 5C/450A was first introduced in 1947.