My first eyeball-gut reaction was 173x half reale. Looking at Trez's sample picture, the 7 and 3 are very close to OP's find. Crazy to insinuate that the die maker might have made the respective dies for the different cobs. Also, the quality of the planchets and dies fell off a cliff in the 1700s after Mexico City transitioned to milled coinage. Almost like the other mints just said F-It, this is good enough which makes finding a suitable example for a collection a real challenge.