Originally Posted by Dan S. W.
You may want to peruse the Webley catalogs of the era for a direct answer. Birmingham makers definitely employed the feature as I have it on my G&S Holloway and another gun that also likely originated in Birmingham.

Thanks. In the Webley & Scott 1922 Wholesale Catalog, I did manage to find one high grade model W. & C. Scott & Sons shotgun offered in BL and SL that did have a concealed top sliding bolt. All other catalog offerings were either the cross-bolt or screw grip rib extension type fasteners.
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