The SP61 was a commercial television RF pentode that became adopted by the military during WWII. We have several variants in the collection, but this is the original livery of the manufacturers and is covered by an external screening metallisation. For mixer use a separate low noise oscillator was designed this was the P61 also known as the VR66.
A special batch, made in 1942, was the source of valves for the Anglo-Irish undersea telephone repeater cable laid in 1944.
With the grid at the top cap, this is the normal pattern for RF and IF amplifier valves of the period. Providing high gain in tuned anode circuit applications.
The rear of the envelope is just plain zinc paint.
Little detail is revealed by looking into the top.
The classic envelope is 32 mm in diameter and, excluding the MO (Mazda Octal) base pins, is 81 mm tall.
References: Data-sheet, 1043 & 1040. Type SP61 was first introduced in 1941.