Before materials like stainless steel became available, gold was used for watch cases because it doesn’t corrode and doesn’t cause allergic reactions after prolonged contact with the skin. (Some people have allergic reactions to metals like nickel and chrome; that’s the reason for gold and surgical-steel posts on earings.) As an alternative to the expense of solid gold, cases were made of brass (or another ‘base’ metal) with a gold layer on the outside, which offered the same benefits. Gold plating is by far the cheapest way to cover a watch case and gives the thinnest layer of gold; I don’t believe that the original Gruen company made any gold-plated watches. Rolled gold plate is made by a process that bonds a very thin layer of gold over base metal. This is thicker than gold plating but thinner than gold-filled. Gruen used this process when they started making lower-priced models in the 1950s. A gold-filled case is a sandwich made from a slab of base metal between two thinner slabs o