Did the 21j Bunn special always come in a gold case or could a railway worker who had to buy such a watch save a few dollars by going for a cheaper case. I have one that I bought as a parts watch and it came in an Illinois Spartin case. It was a non runner and when I took it apart everything was full of coal dust. This suggests case and movement have been together for some time and may have been used in the steam days. The crystal is yellow with age. There are no other screw marks on the case. Could this be an origional combination? Movement no is 4392040 which I believe to be 1923. Case no is 0472542 but not sure when this was made. Your thoughts please.

Illinois was advertising the availability of the Bunn Special grades as "movement only" at least as late as June 1926, after the introduction of the 60 hour movements. Prices were $50.00 for the new 60 hour movement only and in first model 10 Kt yellow gold filled cases at $60.00 or in first model 14 Kt gold filled green or white cases at $65.00.
It appears that the 60 hour movements were available uncased for only a short time, as by August 1926 the ads we have no longer mention "movements only".
BTW, if the crystal on your watch is yellowed from exposure to UV rays, replace it as soon as possible. These old crystals will absorb water from the air and start rusting the hands and the tip of the cannon pinion.
Modern plastic crystals are made of a different material and will not yellow or cause the hands to rust.