The 5686 is a special quality pentode. Here it has been overprinted with the UK government procurement specification CV3612.
It can be used as a Class A audio amplifier to give 2.7 Watts single ended or a Class C RF amplifier up to 160 MHz where the output is just over five Watts.
The CV identification. The screening is seen top and bottom. The top mica is thicker than the bottom one. Thicker mica discs reduce the hole enlargement caused by vibration - hence special quality.
The 5686 etched into the glass as well as USA.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 47 mm tall.
Reference: Data-sheet. Type 5686 was first introduced in 1952. See also1952 adverts.