I have never seen a DT tail with a true timer but the 40s are before my time so? The timer I had looked like an aluminum tube/can with a plunger that pulled out. It had a threaded end on it with a round nut on it so the timer could be set. The other end had two battery tabs on it. I assumed it was for ignition cut off but never really knew how it all worked. I had another plunger type that the fuel line was laced between so when the time was up it just cut the fuel supply to the engine. By the time I got into free flight the DT tail was controlled by slow match. Two metal pins and a rubber band held the tail feathers down with the slow match/fuse between them and the rubber band. When the heat hit the rubber band it broke and the tail poped up and the plane floated down in a sea saw manner. Before I was taught about the DT tail I had lost a couple of planes that got caught in a thermal.