I received a couple more photos. Apparently there is no text on the left side of the action. I believe the markings means the following:

PV surmounted by the lion is Nitro proofing used by FN in Belgium. This proofing was used in the period 1898-1968. (It's very possible that other smaller factories used the same proofing, but the rifle is produced in Belgium for sure.)

The marking in the middle, the Perron, is a final proof.

The Z with a star above is apparently the inspector mark. There are two guys that used Z with a star above as their inspector mark in the time span I find relevant. Mr. Degobert Theodore 1924-1949 and Mr. Luyten Martin 1952-1968 and 1974-1989. I believe there should be more markings, but they could off course be hidden under some wood. A single letter should be sufficient to find the production year.

I believe the last picture was very useful, and I don't think we will be able to get any further without more details available. The rifle is located some hours’ drive away, so I am not sure I'll ever see it live. I still believe the rifle is produced some years after WW2. Anybody have any idea for how long the factories kept supplying rifles in this chambering? At some point I would guess they concluded that .404 Jeffery was a better design?