i used to collect them but my current gf is scared of them, so mine are left in storage right now with my youngest brother.
think the one you're on about is the ring revolver (one shown in the link above is one of the early english prototypes), most are in museums now; so best way to get one is special order from a jewler/weaponsmith.
some of the weapons i've collected over the years have been custom from weaponsmiths.
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