Webley made several patterns of screw-grip actions for both shotguns and rifles. Yours appears to be the most basic of the shotgun actions, with a straight extension as opposed to the doll's head of the more expensive A & W model. You'll find it on page 7 of the 1922 Webley & Scott catalog.

If there is a Webley number on the gun, it wouldn't be on the fore-end loop during this period. It would be on the short rib between the flats and the loop. During this period, it would be in the 62000 to 69000 range. Webley never numbered actions. They assigned serial numbers only when they barreled a gun, and the number was stamped on the barrels. Do I understand you correctly that there are no numbers on the gun at all? If so, this suggests that your surmise might be correct.

According to Brown, Calder was in business in Guild Street, Aberdeen from c. 1894 - 1917. He was at 30 Guild Street c. 1901 - 1905, so this gun should be from that period. The lack of a serial number suggests a small retailer who did not assign his own serial numbers, a surprisingly common practice in those days. With respect to their trade with other gunmakers, Webley provided mostly completed guns ready for sale, and some proven barreled actions in the white to be stocked and finished by the retailer. However, when Webley completed a gun for another gunmaker who did not assign serial numbers, they would usually engrave their own serial number on the guard tang and, of course, stamp that number on the barrels. If this gun has no Webley number on the barrels, and none elsewhere, then Webley neither barreled nor completed it.

I imagine this gun was built on an action that Webley sold to the trade, thus no Webley number, and was barreled , stocked, and finished either by Calder, or for Calder by someone else. It is commonly believed that Webley provided large numbers of screw-grip actions to the trade. With respect to the rifle actions, this was actually rather rare, but may have been more common with the shotguns.
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