Hamilton purchased the Illinois Watch Company in the late 1920s - just in time for the Great Depression. Illinois Watch Co. continued to make models for a few years but Hamilton eventually moved production to Lancaster PA and shuttered the Illinois factory.
Illinois was a quality manufacturer though and had a very strong reputation. Hamilton reintroduced the Illinois brand in 1953 as part of a strategy to introduce using Swiss-made ebauches without risking the reputation of Hamilton-branded time pieces.
Europe was still recovering from the devastation of WWII and European watch companies made very high quality watches at price points that were difficult for US makers to compete with. One by one, American watch companies either closed shop or chose a "if you can't lick 'em, join 'em" strategy of using Swiss-made calibers to lower their price points.
There were a few Illinois models introduced in 1953, including the first automatic models. One of them was the manual winding Nautilus Model A.