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 two pentode sections. The design features a pair of top mica disc. The elevated top mica carries the four filament tension springs. The anodes are strapped together and are connected to the top cap.
The the three grid supports can be seen at the top. At the base a mass of tape can be seen welded to the supports and paralleling the sections. This looks to be a complex hand operation and thus not a quick or cheap valve to manufacture.
The classic envelope is 39 mm in diameter, and excluding the MO base pins is 97 mm tall.
Reference: Datasheet. Type ATP4 was first introduced in 1940.