This planar triode has both Machlett and Raytheon names on the main ceramic section. It is clearly a power valve, suggested by the ceramic, and designed to operate within a co-axial cavity. The frequency is probably around 1 GHz.
The anode fins are machined from a block of aluminium and remain as square section columns.
The concentric connections are, from right to left, heater in and outside of the end connection. The grid is in the centre and the anode is at the left of the picture. The maximum diameter at the anode fins is 30 mm.
The planar envelope is 20 mm in diameter, and overall is 67 mm tall.
Updated April 12, 2013.