I think that idea may have come from the manufacturer, a 1968 catalogue says the same thing (top right in text):
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Also says same about FWB110,they call it 'delayed recoil', feature no. '4'
I think they recon the pellet is out of the barrel before the piston gets to end of travel, maybe possible if there was enough swept air volume? although you'd still get the initial rearward movement as the spring expands.
Don't know how you'd measure it, other than with a super high speed camera.
This from 1978 shows a few Webley Osprey Supertargets in the mix, one got silver.
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as Geezer said it would be interesting to see some 1960's result lists.
Cheers,
Matt