Keystone Watch Case Co. was the major gold fill p.w. case supplier in U.S. Here a 12 size for 1920-40 movements of unusual double hinge back and front glass I think was a demonstrator that has the popular Waltham Colonial movement, with crude dial marking for Waltham, but inner cover marked KEYSTONE WATCH CASE CO. That marking usually for the movement maker. My several Colonial Walthams are in the fairly common thin flimsy single back gold cases that are poor protection without much 14k gold content. This odd example is the best g.f. 12s case I have seen. Apparently not successful? Keystone marked their best g.f. cases like this one, exceeding 25 year wear rating, as EXTRA. I have not noticed this fine Keystone style on any watch so I think is a demonstrator for a not very popular design.???