FWIW;
If the proof marks on your gun are 1.1/4oz then this would have had 2.75" chambers & was likely a model 300.
The model 400 was proofed for 1.1/8oz shot with 2.1/2" chambers.
I had a similar age (ca 1921) grade 3 (border engraved) model 400 that had been sold/stamped as W.& C. Scott & Sons, 55 Victoria Street, London, England (Webley/Scott's London office in early 1920s used for the export business). It had Disc set strikers, ejectors, screw grip, 30" barrels, Nicely figured wood & weighed about 6lb 10oz. It was lovely to hunt with.
A few things I found out about these "Proprietary" model (300 & 400) guns of this period. They were supposed to be simplified basic working tools meant to stand up to rough treatment and local mechanics far from the "civilized" world.
W&S used provisional 1925 proof markings from at least 1921 onward. A model 400 would (usually/always?) be proofed for 1.1/8oz shot which meant 2.5" chambers in a light game gun. The model 300 was identical except being proofed for 1.1/4oz shot with 2.75" chambers and being a slight bit heavier. Neither of them were marked with model #.
The Screw grip was one of the features of these models.
There was a noted English trap shooter (name escapes me for the moment) who used the model 300 extremely successfully. He apparently liked the idea that he could routinely use this low-class off-the-shelf working man's tool to beat up on the proper gentlemen with their bespoke purpose-made London Best guns. He had his collected Medals covering his gunstock like a Gypsy which apparently gave him great pleasure & his competitors considerable irritation.
This info was gleaned from the Crawford/Whatley Webley & Scott book and cross-referencing with some W&S catalogs & product sheets.
YMMV
