Off t'bed now so the 'in a nutshell' version...
Basically what it says on the tin... a mobile control tower.
Used by both USAAF & RAF during WWII and beyond.
They were not assembled to any particular pattern or indeed on any particular vehicle...simply a case of whatever was available. Occasionally GMC's, frequently british built army office/living vans and everything downwards from a gypsy caravan to a jeep trailer and even a rickshaw...no really!!
In dusk, poor visability, dawn or just because of the lay of the land the Flying Control trailer/truck was in essence an extra pair of eyes at the far end of the airfield from the tower. Driven to the far end of the active runway it would, depending on conditions and visibility, control the departure of aircraft (& return later) where the tower was unable to do so. Fitted with The necessary radio equipment (though not always) to communicate with the aircraft and utilising signal lamps and flares. The main objective (on takeoff) to see the aircraft off the runway safely and set the next one on its takeoff run as soon as the 1st plane was airborne (or at least past the point of no return!). As part of the 'make it up as you go along' or "bodged" as I like to call it, school of engineering the trailers/trucks were often fitted with observation domes for the F/C crew to observe the goings on. These were usually cracked/fogged or generally U/S plexiglass astrodomes, nosecones or gun turrets. Usually painted in high visibility chequer pattern, black and white/black and yellow. Not to be confused with the similarly painted 'Follow Me' taxi-way jeeps who escorted the bombers/fighters to and from their hardstand (although operated by the Flying Control crews again)...
If there's anything you want to know shout up... I might know the answer... might...:???