Gruen Hunter case
Your case is rather rare, and appropriate for any 16 size Hunter movement from the 1898 to 1911 period.
It may not make sense, but Gruen sold cases to jewelers and jobbers in standard American sizes, for use with their competitors movements. From the 1870s through the early 1900s, the major manufacturers of watch cases aligned themselves in a trust, and gave preferential pricing to the big American makers (Waltham, Elgin and eventually Hamilton). This is what drove John Dueber to buy the Hampden Watch Co., but that's another story...
In order to guarantee their supply of watch cases, in 1898 Gruen bought the Queen City Watch Case Co. of Cincinnati. Queen City was a relatively small but high quality maker of solid gold watch cases. Gruen changed the company name to "Gruen National Watch Case Co.", and would operate it as a subsidiary until 1911. After that, the case works were incorporated into the Time Hill factory, and their cases are simply marked "Gruen". GNWCC cases are usually seen in smaller (0 or 6) sizes, and are often beautifully engraved, but they also made larger cases. To my knowledge, all Gruen National cases are 14K or 18K.
There were Swiss movements made and imported in American sizes, notably by Omega, Patek Philippe and Vacheron Constantin. Gruen also imported a modest number of 16 size movements, some of which entered Canadian railway service. Any of these would be appropriate for your case, but so would an American movement from the time period. If you want to find a Swiss movement, make sure it's one with American style winding, where the stem is part of the case, not part of the movement.
Regarding Gruen hunter cased watches, I am not aware of any Gruen produced hunters after they consolidated production in the Biel factory around 1908. I do not believe there were ever any VeriThin marked hunters, but Gruen did subcontract with several makers from Madretsch, and there were some hunters produced by those makers.