Two candidates (since I can't make out any image or letters on either side): Fernando VI (1746-1759) and Carlos III (1759-1788). Both kings issued Mexican 1/2Rs measuring 16mm in diameter. You might check the weight. The coins referenced above each weighed 1.69 grams.
Thanks for the second pic.
Don.......
A light rub with some olive oil might bring out some features.
hi, I just took coin from olive oil soak that's been going on for about 8 years now.