I bought this from an auction and it said the movement was removed from a gold case. I hate to see these without a home so I re-cased it in a 3xt Ill Keystone case.
Rob knows this but for those of you who don't: Illinois made a series of similar "extra thin" model 1 watches, Models 435, 437, 438, and 439. The same movement was remodeled to make Illini Grades 538 and 528. Hamilton remodeled Grade 528 after it acquired Illinois to make its Model 400. All of these watches were high grade gentlemen's watches, most originally cased in solid gold. None of them were made in large quantities. Some are quite scarce. For example, only 310 Grade 435s, the 17j version, were made. 2,060 Grade 437s, the 19j version, were made. 1,130 21j Grade 438s were made. There were 330 or 370 (depending on one's source) Grade 439s, the 21j "extra" version that Rob has rescued.