400 NE, I agree with you on the dating of Dustin's gun. Does your catalog specify that that particular gun is a screwgrip?
Yes, it does. Like I said, I don't pay nearly as much attention to the shotguns, and I haven't handled that model in a while, but I don't recall it having visible machine threads on the spindle. The catalogue describes it as a screw grip.
I have a couple photos of screwgrips, and both have the word "patent" around the screw through the top lever--and they both also have a small screw set into the rear of the large, spindle screw. Without being able to see whether the spindle on Dustin's gun is threaded, (Dustin, you should be able to see the threads in the slot in the standing breech), from the photos at least, it looks as if it could be just a standard Scott spindle with a 3rd bite.
While I have no wish whatsoever to start an endless controversy here, your above para pretty much outlines the crux of a very popular misconception about these. The term "screw-grip" is commonly used today as a nick-name for some screw grip models that have machine threads on the spindle that are visible through the standing breech. That definition is a modern invention, and one that I can find no evidence that Webley ever used.
In doublegun nomenclature, a "screw grip" is a fastening system used in three different ways. The first was Henry Jones's screw grip from 1859, which fastens the barrels to the action via a T shaped screw head which engages angled slots in the underlug. Turning the underlever tightens the "screw". Wilkinson Sword's Screw Grip Fore-end Fastener of 1866 (the side-swinging lever type seen on the Army & Navy rifle above) is a direct crib on the Henry. The lever bears against an angled cut on the loop, "screwing" the fore-end iron tightly into place as it turns. Webley & Brain's 1882 Screw-Grip Top Fastener works in exactly the same way - an angled extension on the spindle engages a matching cut on the rib extension and turning the toplever "screws" the barrels down tightly. The term "screw grip" is a description of how the fastening system works, not of a specific characteristic of a few models of the type.
Looking down on the receiver from the top, the top levers on my Army & Navy pair (made by Scott in 1933) don't look any different than on this gun. But mine have Greener crossbolts, definitely not screwgrip guns.
After the merger in 1897, the P. Webley and W & C Scott lines remained more or less separate, even in the catalogues, until the Scott line was dropped in 1935. The screw grips were Webley models and the crossbolt guns were specific Scott models.
