Under the Registration of Firearms (Ireland) Act 1843, firearms held were required to be registered by area and a registration mark applied.

The marks generally comprised 2 letters and 3 or 4 digits, so for example KE1234 would be the 1,234rd gun registered in Kerry.

Thinking about it B10 is probably in the wrong format for an Irish mark.

There were similar schemes in various British Colonies.

The rationale seems to be accountability if the guns feel into the wrong hands.