I have two muzzleloaders having laminated steel barrels with London proof marks:

In one case the barrels are marked on the underside with a definitive proof mark, the gauge (14), a view mark, a provisional proof mark, the gauge again (14) and at the breach end "B10".

The other ML has barrels similarly marked except without a provisional proof mark. At the breach end the barrels are marked "60" or "09", depending which way is considered right side up.

Does anyone know what the mystery numbers at the breach end signify? Thanks.

As an example, here are the markings on the underside of the barrels of one of the muzzleloaders, i.e. the one with the mystery "O9" or "60".

Here is the 2nd set of barrels with the mystery "B10". I would welcome any info on what the mystery markings mean,
including if the mystery mark sheds any light on when the barrels were proofed.

From the positioning of the 09 or 60 marks close to the witness marks aligning the barrels and breech plugs my guess is that they are assembly numbers to make sure the right plugs were threaded into the correct barrels.

The B over 10 might be a local colonial (or Irish) registration mark.

They are not part of the Proof marks.

Parabola, Why does Irish come to mind as a possibility?

The gunmaker who made the barrels was John Mullin, born in Ireland, who moved to NYC in the 2nd quarter of the 19th Century and built guns in NYC with London and possibly Birmingham proofed barrels.

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.

Thanks, Parabola. The "B10" remains a mystery mark which stumps everyone I have asked about it.