Yeah, the G20/15D RC was still on sale "mid-seventies" 'cos I bought one!
IIRC Super Tigre started phasing out the G20 when the G15 was released, but the G15 Diesel was generally considered to be not as good as the older G20/15D so, after a period of hesitation, the G20/15D (and glow) became available again, plain carb and RC, particularly as, during the 70's, the 15 glow had become the favourite FAI combat engine in Europe.
I've had several G15 diesels (still have one), and I prefer the G20/15D, particularly the "1965" crankcase version.
I'm still doubtful about the engine on sale that started this thread, but it's just a sort of "gut feeling" on my part, nothing that I can substantiate or argue about!