Perhaps the Milburn stamp was made for them specially, they didnt mfg shells but may have had a sporting good store or something, but the actual shell was made by Kynoch, like they did on buttons sometimes put in a retailers name, but scovilles or some other one actually made the buttons
From Wikipedia:
Kynoch was established in Witton in Birmingham in 1862 by Scottish entrepreneur George Kynoch when he opened a percussion cap factory in Witton.[1] After the First World War many of the UK ammunition and explosives manufacturers were brought together under Nobel Explosives to become Nobel Industries, which was a founding element of Imperial Chemical Industries Ltd (ICI) in 1926. Once Nobel Industries, including Kynoch Ltd, had merged to form ICI, the original Kynoch factory in Witton became the head office and principal manufacturing base of the 'ICI Metals Division'. Kynoch along with names such as Eley became brands of subsidiaries.
With the standardization of cartridges across the Western powers and a general downturn in ammunition requirements, the sidelines in sporting cartridges were discontinued by Imperial Metal Industries (IMI) in 1970. IMI became independent of ICI in 1977 still producing rimfire and shotgun cartridges for the sporting markets. The more economically viable production of shotgun and rimfire ammunition continued. The Ammunition Division was incorporated separately as Eley Limited in 1983. Kynoch also produced motorcycles.
A British company has licensed the Kynoch name to produce the older range of ammunition with modern materials.