This watch looks to be a "Southamption," a generic case also known to house Elgin and Illinois movements. See Fred Friedberg's first book, pp.195-96, for more information. When I get a chance, I'll take a look at some of my jobbers' catalogs and see whether I can find a Waltham pictured in that case.
(By the way, does the marking inside the case back happen to include a "W"?)
Thank you so much. Yes, there is a "W" on the inside case back. It is right after the "6/0". See image attached.