awesome find!!
this looks like 18K or 22K and of the late Georgian period (late 1700-early 1800s). it is quite worn--the shield shape on top would have once had an engraved monogram probably.
the marks sort of look like birmingham but they seem a bit off (not necessarily a bad thing...). the duty mark doesn't quite match any of the ones i'm looking at in the book (could be a variation). also it's odd that a date letter is not present--this certainly would have been included if the maker went through the trouble of putting a maker's mark, city mark, and duty mark... this could be because of laxer marking standards or because the ring was sized or repaired at some point, thereby obscuring or removing marks (can you detect any seams from a repair or sizing job?)
also the typography of the "MP" is a bit weird for english hallmarks.
anyway, sometimes when you see pieces of this period with english-looking hallmarks that seem a bit "off" it is actually chinese export. they used "pseudo-hallmarks" in imitation of english hallmarks. the typography of the "MP" is similar in style to marks on some chinese export wares i have seen--the lettering is more stick-like and without serifs (like the characters in their native language). i know this from my somewhat limited experience with chinese export silver.
sorry to disagree with rodgerdodger but it is definitely not mexican or electroplated. it is solid gold and almost certainly late 18th/early 19th century.
at the very least, a handsome English Georgian signet ring, but if it is chinese export it could potentially be worth more.
don't scrap it! it merits more research.
congratulations!