That's because it was struck in Bolivia instead of Mexico. See Krause #53 page 93 in my Third Edition. It is actually JR but "I"s and "J"s were interchangeable at that time.
The reason you can't find it may be because you are looking at Mexico City mintage--which does have the FM assayer's initials. What makes this coin different is the mint mark--it's from Potosi and the assayer's initials are JR. BTW, the date on your coin appears to be 1776, not 1775.
Don......
PS: Paddy beat me to it, congrats!