Just judging off owning piles of uniforms this button dates to probably roughly the WW2 era. Can't recall seeing any Waterbury buttons on a jacket like that on anything newer.
Does the backmark say Co. or Co’s? In the early 1940’s Waterbury changed their name from Company to Companies in order to reflect their expanded product lines. So if it says just “Co.” (singular) the it’s before then, and if it’s plural it’s later.