The Webley "screw grip action" has nothing to do with an underlever..."screw grips" were mostly used on box locks
You're confusing several different kinds of screw grips. There was no "Webley screw grip action". Webley didn't invent the screw grip, and the visible threads on the spindles of some Webley models have nothing to do with the term.
The screw grip was first patented in Britain by Henry Jones in 1859 - an "underlever screw grip for breech loading actions" - typically referred to in the trade as "Jones underlever" or "lever over guard" as Lagopus correctly stated. The Jones is a screw grip action. It's primary (and only) locking system is a screw grip.
The same rotary locking method was then patented by Wilkinson Sword Co. (James Wilkinson) for fastening fore-ends in drop-down barrel action guns. The 1866 screw grip fore-end fastener patent (the familiar side-swinging fore-end lever) was by a Wilkinson workman, which made it Wilkinson's property. Wilkinson Sword Co apparently sold the patent to Rigby before it expired.
After both of the previous patents had expired, Thomas W. Webley and Thomas Brain designed a rotary locking THIRD FASTENER for drop-down barrel action guns that was a direct crib on the previous screw grip patents. In the 1882 Webley & Brain screw grip top-fastener patent, primary lockup is accomplished by conventional (Purdey) sliding double underbolts that engage corresponding lugs milled into the barrel lumps, with a rotary locking third fastener that engages a rib extension. The Webley is a conventional sliding double underbolt action with a rotary (screw grip) top fastener. It isn't a screw grip action, while the Jones underlever is. Webley made a great many sidelocks with their screw grip top fastener (their W & R model was made for dozens of other gunmakers) in addition to several models of their famous boxlocks.
Here's a Webley W & R Model (sidelock) built for Army & Navy:
http://www.cabelas.com/cabelas/en/conten...erarchyId=11655Here's a Webley A & W Model (boxlock) for Charles Lancaster:
http://www.cabelas.com/cabelas/en/conten...jsp?hierarchyIdNote that the same spade head was used for both models.
Here's a Webley A & W Model for William Evans:
http://www.cabelas.com/cabelas/en/conten...erarchyId=11655Note Webley's famous "winged bullet" trademark on the flats typical of early guns built for the trade.
Here's a Webley W & R Model for William Evans:
http://www.cabelas.com/cabelas/en/conten...erarchyId=11655