Here's a previous answer from Mr Skeues to the same question: Hi Guys, The first batch of 28 ga guns that we ordered had that F on the lug. They came that way from Japan. I think the manufacturer misread an accounting code we used for computer codes. I do not know what the I means.

I have a Repro 28-.410 combo and the 28ga barrels have the same lug markings: I and F
in the lugs-serial 28-00952. .410 lugs do not have those markings.

I just picked up a NIB Parker Reproduction 28 gauge, guys.

The serial # is two digits, under 50, and has I - 2 - F on the lug. It came as a one barrel sale, but could the 2 mean it was the #2 barrel of a set? (I wish it was stamped OO instead of F, but...?)

It's a tight little gun which will be used for birds this fall over my Brittany's.

Great conversation on this site with the opportunity to learn a lot..!

Thank you!

Wes