I think you can say with almost certainty that its a VCS3/Synthi A - based only on the synths that JMJ had at the time. The effect is an LFO (on VCS3 this will be OSC 3) modulating either one or both of the other oscillators, most likely set to sawtooth waveforms. The get the effect the LFO speed will be modulated manually, either by the rate (Frequency) dial within the oscillator, or it may be mapped to the joystick. Often you'll find that filters play a part too, starting dull, brightening up, then dulling down as they fade away. This again will be done manually, either by adjusting the cutoff knob or again, assigning it to the joystick.
The joy with the VCS3/Synthi A is that both the LFO frequency and the filter can be assigned at the same time. This style of sound can be achieved with almost any analogue or virtual analogue synth with one assignable LFO and at least one oscillator.
Having said all that, GeeGee may also be correct in saying that it's the Trapezoid Envelope - on the EMS gear it isn't really an envelope in the terms of ADSR, but more of a trigger. The Trapezoid Envelopes are unique to the EMS gear but I think Doepfer have made a trapezoid module for their A100 modular system.