Utterly sweet. Reales circulated along with USA coins, in the early days of California statehood. But by the mid to late 1850s, the SF mint had finally gotten the coin shortage abated. Still though, I've even seen reales surface from sites that didn't get founded till the 1860s, meaning some continued to circulate even into the 1860's.
Congrat's the find. What type site did you find it in ?
In 1979, I found a copper 1812 Mexican Reale just outside of Merced California. I also found a small copper bowl near the same hole. Back in those days I was using a Bounty Hunter TR550. I still have that detector, but now I used the Garrett AT Pro.
The fineness of the silver was measured in the medieval system of dineros and granos. 12 dineros was pure silver. Each dinero was divided into 24 granos. If totally legible, the inscription under your radiant cap would read, in part, 10Ds. 20Gs; or .902777 fine.
A dime is 21.2 mm wide and a 1 real coin would be 20mm wide; close enough to call it a 1 real coin.