Canuck
·Back in the day when silver coins were struck for general circulation in the U. S., they were .900. Fineness varied by nations though so don't know for certain what "coin silver" indicates.
It has long been my understanding that U S coin silver is .900, and English coin is .800. How a case on a U S made watch has a .800 case is odd to my way of thinking.
For more information of precious metal marking, check out the following.
https://www.vintagewatchstraps.com/swisshallmarks.php