Mike feels there might well be a technical connection between the Simprop Single channel propo set, the Super 1, from 1969, (Peter Gruchot sent me a copy of the original Simprop magazine ad that is reproduced below) and teh GEM 1.
Mike knows that the Mainstream produced/ assembled versions of the Simprop Digi 5 and 2+1 have a large amount of electrical circuitry similarity with the later produced plastic-cased GEM range of sets.
The rudder stick has a sort of proportional control over the servo, which has a distinct Simprop look to it, the motor servo is a sequential and keeps rotating and will cycle the throttle control between slow and fast, releasing the rocker stops the "servo". The rudder servo also flaps from side to side whilst all the throttle action is taking place!
Anyway, if anyone knows more, then please chime in, I fear all my contacts in Harswewinkel at Simprop who might have been able to give a definitive answer have long passed on.
so here is a reversed (!) unboxing sequence of photos for those who may never have seen one of these little GEMs (!)

