Very interesting, Mick.
This thread is also a handy reminder that until the US “adult airgun” thing of the early 70s started by Bob Law and then Bob Beeman, and taken up a few years later by AGW, the efficiency and power of springers was largely guesswork, including for the makers. Because:
1. No-one had access to chronos.
2. If it worked, no-one cared about the maths or the design.
3. Why spend cash doing the maths/physics?
4. Airguns were airguns, not military sniper rifles, so who cared?
5. Existing designs in production. Just keep making them. And they sell.
The realisation that long-stroke was a better way of making power than cylinder bore size began (a bit) with the FWB Sport in 1972, but only really hit home with the HW80 compared to the HW35. The Diana 35, compared to the 27 which comes fairly close in power, tried to have higher power primarily by increasing the bore, not the stroke.