Hammerless Jones underlever DRs are fairly common. Purdey built hammerless Jones underlever DRs as snap actions at least as late as WWI. Blixen used one in .500/.465 for a time.

In double gun nomenclature, a "screw grip" is a fastener that operates on the interrupted thread principle and is activated by a lever. Sorta like a bayonet mount camera lens, a partial turn of the lever fastens, and a partial turn in the opposite direction unfastens - hence the term "screw grip". It was commonly used as a primary bolting system, a fore-end attachment system, and a third fastener system. The Jones UL Screw Grip Action patent of 1859 uses a screw grip to engage the underlugs. The fore-end fastener you referred to is the Screw Grip Fore-end Fastener. This type was patented by a workman of Wilkinson's (I seem to remember his name as Harvey) in 1866 and was, thus, the property of Wilkinson Sword Co. The T. W. Webley & T. Brain Screw Grip Top Fastener Patent of 1882 is a third fastener on a dolls head extension, and was obviously a direct crib on the Wilkinson patent.

Most of the Webley built screw grip double rifles also use the screw grip fore-end fastener.
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