Hi Redoak
It is most likely that Leeson would have sourced the action forging direct from Webley when he required an action fitted with the Webley Screw Grip, especially if the action was to make use of the Dolls head type extension rather than the Blade type extension. As you are no doubt aware forging anything, particularly a shotgun action is difficult at the best of times but the cutting and profiling of the slot to take the Dolls head required a very special broaching tool setup which would have been beyond the normal tooling costs of any gunsmith, hence you are quite right that the action forging would have been purchased from the trade, in this case almost certainly Webley. The barrels could have come via a number of sources, potentially from Webley or direct from a specialist barrel maker or indeed via Leeson himself. Webley would have supplied the matching Dolls head which is always attached to a very short rib section which is about 21/2” to 3" long and soldered to the main top rib during assembly.

As you are no doubt aware, once the A&D Boxlock came along as a basic design, many of the larger Birmingham trade suppliers, Webley being just one, tooled up to make this action, Webley of course developing the Screw Grip mechanism and from here on almost certainly supplied any gunsmith with the action should that company wish to incorporate the WSG design into their own gun. Some "makers" would have taken a complete gun, perhaps finished, all the way through to a simple "in white" forging or parts for the gunsmith themselves to finish to their own taste. W R Leeson tended to do a bit of both. If it was an inexpensive Boxlock he might have the gun more or less finished by the Birmingham trade. If it were a more expensive Boxlock or sidelock making use of the WSG mechanism he might purchase parts "in white" and build the gun from scratch.

Thanks for your question, appreciate your interest, all the best for now