Hello,
I already read the topic You suggested, no question, there are some similarities.
To tell You the truth, I do not understand too much about proofmarks. I posted these photos to a Hungarian forum and they told there, the stamp FN should stay for the Fabrique Nationale, Belgium. They informed so, because there were some sets of shotguns, which were manufactured with imported Belgian barrels for export to the U.S. after the World War.
The problem is, that nobody can tell (enough) infos about the Fegyvergyár/FÉG in Hungary, because it stopped (went bankrupt) at the middle of the 90's and all databases of the factory lost and as far as I heard, the old 'gunsmiths' of the factory are a little bit incommunicative.
To the Nitro proof: as I know, it is true, what You wrote, there were a period, when the guns had to be controlled again and again on nitro, but nowadays they make it without any proofmarks or stamps.
Now, a Hungarian gun had to be checked every ten years on functionality and security. As far as I know, they check the status of the gun, if there are any visible mistakes/damages, they check the pressure (?), which is needed to pull the trigger (sorry, but I do not know the exact word for this) and they shoot with much more stronger loads to check the endurance of the gun barrels. After that the gun gets a plastic card (but no marks any more), which is like a proof-licence for the gun for the next 10 years. If somebody misses to renew it within 10 years, he can get big problems.
Back to the point: where can You see, that the manufacturing date was 1930?
I would be very happy, if I could read something about all of the proofmarks on this weapon, because on the internet there are not so many online. I hope, this is not too much expected :-)
Best regards,
Zoltán