Maybe not Unobtainium, but at $861… definitely Unaffordium.
Here’s the drawings from Prideaux’s US Patent application which differs quite markedly from the production item … however, a Patent is only for the design or design principles, it need not be an exact replication of the object itself, to avoid giving ideas to competitors;
Here’s a picture showing the loader disassembled, with rather a nifty fixture to show the operation of the cartridge retaining “fingers”;
Prideaux was a prolific inventor but his inspiration for his military designs, especially his early machine-gun links, sprang from his box latch;
Trench warfare on the Western Front had shown up how sensitive the Vickers gun was to the canvas belts then in use getting wet or soiled with mud (or worse) … hence the development of weather and grime proof metallic link belts.