The button appears to be a pewter button of the manufacturing style more often associated with those of American Continental troops. The large integrally cast pewter shank, smaller size face, prominent mold seam, and simple Arabic type numeral, all point towards what is frequently seen with our Continental buttons from the American Revolution. British Loyalist buttons of the period, and those of British Regulars, usually have other distinctive features in pewter, tombac, and brass, such as (but not limited to) pie crust style border designs, iron wire shanks (on pewter), etc..
Interestingly, according to the listings in
Military Buttons of the American Revolution by Don Troiani, the 5th regiment buttons of both the Continentals as well as British Loyalists & Regulars, display a Roman numeral 5 (V) rather than an Arabic 5 as we see on this unusual example.

The stylization of what I'm noticing in this particular numeral 5 on the button above, with the proportionately large looping semi-circle bottom and very short tilting top bar, is very indicative of what is seen in other Arabic numeral 5 depictions of Colonial era. Quite possibly this particular button, may be a variety previously unrecorded or unknown, of the 5th Continental Regiment. There is a known example of the 25th Continental Regiment button in pewter, and the numerals are Arabic, displaying a 5 of very similar shape and style.
Considering the potential rarity of an unknown variant coming to light, certainly warrants forwarding information and photos of this particular button to the leading experts on the subject.
CC Hunter