More information? here the original patent information: Swinburn patent . I have 2 of these - 1 is a basket case, needing to be resurrected. The other is a sporting rifles with all the bells and whistles in 577-450. I don't shoot it all that much, though. That's the one the new cocking lever DoubleD mentioned went on.
A nice Martini variant, but conceptually and technically inferior to the real thing - especially the extractor is a real Rube Goldberg contraption...
Still, a nice rifle to have in its own right. And much rarer too.