Julia Casey submitted these notes on the "Crowned F" counterstamp. Thank you! -Editor

A gun's "foreigner's mark" is a stamp or symbol placed on a firearm to indicate that the gunmaker was not a member of the London Gunmaker's Company. This mark was typically a small "F" or "F" under a crown, signifying that the gunmaker wasn't part of the official guild.

Elaboration:

London Gunmaker's Company:
In the 18th and 19th centuries, the London Gunmaker's Company was a prestigious organization that regulated the production and quality of firearms in London.

Foreigner's Mark:
If a gunmaker wasn't a member of this company (and therefore wasn't subject to their rigorous standards), a "foreigner's mark" was placed on their firearms to indicate this.

Example:
On the Met's pair of flintlock pistols, the foreigner's mark is an "F" beneath a crown, along with other markings like the gunmaker's initials and the London proof marks, according to the Metropolitan Museum of Art.

Significance:
This mark served as a way to distinguish the work of non-members of the London Gunmaker's Company, potentially affecting the perceived quality and value of the firearm.

Brunk-F-3 Crowned F Counter stamp 1749 English Farthing. Brunk does list this under English C/S pieces as Crowned F and this one is similar while on a different host coin. I think a Proof mark from a gunsmith or arms company.