Nigel Brown's book was not a great deal of help as the publicly available serial numbers are only for shotguns, Watson Bros numbered their rifles in separate serial number blocks and the rifle & handgun records are not public domain.
The only hint is that rifle #11280 was made in 1883, so yours is 2286 numbers away. Late 1890's maybe….Quen Sabé?

Be interesting to hear what the actual records details are.

- Mike