Jamie,
Perhaps I'm telling you something you know.
That's a Prideaux loader. British Patent 22653 of 1914 and the inventor's title was,"Prideaux's Patent Instantaneous Revolver Magazine." It was designed for .476, .455, .450 and .38 calibers. They were privately purchased by officers up to the closing weeks of World War 1.
I have one with an RFC Officer's .455 2nd model H.E. that was embellished by Alexander Henry & Co. of Edinburgh and has an additional cylinder, numbered to the gun, in .45 Colt
Frank S.