A button's backmark being written in "plain block" lettering means late-1800s-to-present... literally only a couple of exceptions to that rule. Before then, "serifed" lettering was the rule... or in a few cases, Old English Script. For anybody here who doesn't already know, the drawing below shows serifed lettering with the serifs in red. Due credit is given to our esteemed fellow ID-helper Big Cypress Hunter for creating that educational drawing.
I recall reading someplace long ago that some US Navy buttons were manufactured in Britain during World War One, with that distinctively British "Fine Quality" backmark.