I remember those foam delta models that were built for the military so they could save money blasting away with 30mm instead of expensive 50mm.

I think the guys from RS Radio Systems were in on that project (small orange transmitters and flight packs in the late 70's) their radios were the size of Cannon stuff at the time. The original ones used K&B/Veco .61's with the name ground off the bypass.

Hard to tell what engines were used on those target RC models over the years.