The ATP4 is a screened RF output pentode with a total anode dissipation of 5 Watts. It functions as a single valve but is formed by strapping two sections in parallel.
A hand written note on the PDF data sheet says that the ATP4 was used as the the PA valve in the WS18 set. The valve line-up was VP23 (ARP12CV1331) and HL23DD (AR8 CV1306).
The AT in the designation stands for Army Transmitting and this valve was made for British Army use. The MO base indicates that this was supplied by Mazda.
The twin anodes have no carbonising to improve heat radiation efficiency.
One anode face on with the grid supports appearing above the top mica support.
The fingers on the lower of the two top mica discs grips the glass envelope. The filament tension springs indicate a directly heated valve.
The classic envelope is 38 mm in diameter, and excluding the MO base pins is 99 mm tall.
Reference: Data-sheet. Type ATP4 was first introduced in 1940. See also1940 adverts.