The ARP36 is the UK Army designation for the commercial video RF pentode from Mazda, the SP61. The RAF called it the VR65 and it was used in the airbourne radar Gee. With the advent of the CV register it was renamed the CV1065.
These military versions were all 6.3 Volt heater versions of the original 1938 SP41 with the UK standard, of the time, 4.0 Volt AC heater.
The demand for efficient video pentodes for TV came a year or two earlier in Britain than elsewhere. High gain-bandwidth product was a requirement for TV with transmissions above the top of the HF band at 45 to 50 MHz. Mazda had an excellent reputation and track record in valve design dating back to the AC/Pen and the SP41 and SP61 filled vital needs in military radar and high-frequency communications during the late 1930s and early 1940s, until UK (and later US) production of EF50s was sufficient to meet demands. Moreover, one reason for the huge demand for EF50s was that they had short lives and needed frequent replacement. By contrast, the SP41/61 were virtually indestructible.
The reverse is plain zinc paint. This exhibit shows many signs of corrosion due to storage in damp conditions.
The classic envelope is 32 mm in diameter and, excluding the MO base pins, is 83 mm tall.
References: Datasheet, 1043 & 1040. Type ARP36 was first introduced in 1941.