The proof marks are not British. These marks look like crossed swords, not sceptres. Note that there was no mark at the 6 o'clock position where the letter "P" would have been. Also, there is no view mark present, which was always used in conjunction with the Definitive Proof mark. The "58" would certainly seem to indicate caliber, but the British did not begin marking bore size (marked between the Definitive Proof and View marks) until 1855. I don't think this piece is that recent. Also, bore size was marked in gauge, not caliber, until 1887. Likewise, unless this piece was later converted from flint, I don't think it's old enough to have been proved privately in Britain prior to 1813.
