I wouldn't consider that case as being too modern.
The style is similar to designs from the 1920s through to the 1950s, the corrosion on the case may also indicate a bit of age.
Interestingly, marriage watches weren't always regarded with disdain (disdain created by the "Ukranian Specials").
As fashions changed and the pocket watch was seen as non-de rigueur by the fashionistas, and as something to be repurposed by the thrifty, many pocket watch movements were put into contemporary wrist watch cases.
I think this was especially prevalent when times were hard and waste was scorned, during the years leading toward (and including) the Great Depression.
But like many horological mysteries from the past, this one shall remain a mystery.