The barrels are indeed 3 Iron "American Flag", found on Ithaca Lewis, Minier and Flues Grade 4s and occasionally on other U.S. maker's barrels.

It can definitively be dated by the muzzle over bore dimensions to 1910 to 1921 (there is no lettre annale); probably immediately pre-WWI
The 12 in a diamond was used until 1924
It does not carry Smokeless Powder proof which is not uncommon on guns intended for the U.S. market, and for which there was an additional charge at the Proof House

Very tight 12g bores at 18.0 mm = .709". IF there is choke on the R it is < .008". L 17.4 = .685" so choke .024"
What are the bores now?

[Linked Image from photos.smugmug.com]

The upper 3 letters I believe are JDH which we've found on Lefever G & F grades with damascus barrels. Unfortunately we don't know the identity of that tube maker
The lower mark may be PJ? FJ? What do you think? Maybe chalk it?

BTW the JN may be the lockmaker who worked from home.

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