Per my copy of "The complete guide to watches", #32 from 2012 Zenith dates back to 1875 so pre WW1 production would have all been pocket watches such as yours. By 1920 they had made 2 million watches. Considering the wear in the case engraving it was carried for many years by the owner I suspect. Listed prices circa 2012 start at $185 and top out at 35K for a watch with maximum complications in mint condition so apparently not junk. Any quality marks on your case or the movement such as jewels count or case material marking?