The Allcocks Flick'em is so like the Rapidex I think it more likely that Youngs made them and badged them up for Allcocks, just as they did, with different paint jobs, for Sealey and Millward.
If the link works, here's a Hardy "Eureka", sporting the release system I'm on about, same as Lee's Sowerbutt and the Homer "Flick'em", which \I think is a bit older than Allcocks offering of that name.
Smooth with a capital "Smoo", but I'd be scared to use one for fear of grit, grot or grime getting in the groove and ruining the magic.