That's a good question. That mark certainly was not applied by any proof house. My guess is that some bubba wanted to "upgrade" to the appearance of nitro proof to deceive the unaware.

You should get a reference work on proofmarks. I would suggest The Standard Directory of Proof Marks, by Gerhard Wirnsberger or British Gunmakers, Volume 1 - London by Nigel Brown. Brown's excellent discussion of British proof marks begins on page 260 of that volume.

Your rifle never has been submitted to nitro proof; neither when originally made, nor later.

If you are shooting any loads that resemble full nitro loads you are literally playing with dynamite. Your rifle is for black powder or black powder equivalent loads only. Sadly, you can argue the point until the cows come home, but it doesn't change your situation.

Curl

P.S. I'm very sorry to have to post this information. I know it goes against your genuine understanding of this rifle. Nevertheless, it is what it is.