The Act brought the hallmarking of watch cases into line with the hallmarking of other imported gold and silver items. A small number of silver Swiss watch cases were assayed in England in the years to and marked with British hallmarks. I am not sure when this tax was discontinued, but the case back in the picture here suggests that it started a trend for importing bare Swiss watch movements and putting them into British made gold cases that continued long after the Great War had ended. This seems to be prevalent on cases with the German crescent or half moon and crown. Stockwell and Pringles acted as Assay Agents for a number of Swiss companies who did not have British based offices, so cases that they would have been responsible for having hallmarked if they were imported into Britain were likewise stamped with their sponsor's marks during manufacture. Before the Great War - London was used by many Swiss companies as the route by which they could access the large market of the British Empire, through subsidiaries or branch offices in London with English speaking staff.
This required all precious metal watch cases made in Switzerland to carry a mark to identify the case maker.
British Import Hallmarks in Watch Cases
Each state, city state or city had their own stamps, or "Feingehaltstempel", for gold and silver. For instance, on silver instead of the base of the shield around the date letter having a small point it was rounded. Three standards of silver were recognised,and The marks shown at Merchandise Marks Act are so rare that there may be no watches still in existence carrying them; if you find one, please do let me know. Silver Care.