WOW! Thanks for all the kind replies, I didn't expect so much info so fast. I definately came to the right place!
Shootist43: funny that you mention Jesse James, I have been told I look like the identical twin brother of the real Jesse James (the first picture you see on his Wikipedia page), altough I would have preferred looking like the Jesse James from the Brad Pitt movie
Major Khan: thanks for all the info and the pictures with a lot of additional info you posted, will print them on Monday at work. It seems to me my particular FN rifle predates the rifle from the catalog you showed, the lack of screws in the receiver to fit scope rings is (at least to me) an indication of that (you can see the holes in your last picture). So a 1950's rifle seems very plausible, and it's great to hear that happened to be the golden age of FN, it's something new I learned today.
Interesting that you also mention the floor plate, I have absolutely no fear mine will be "jumping loose" while shooting as I can't seem to open it. I can press the spring loaded button just fine, but the floor plate doesn't move. I could try to pry it open when holding the button down, but at the risk of damaging the stock, so I don't think I will try that. I could also take the screw off, but also here I don't want to risk damaging a screw that has been fixed in place for over 60 years. All the screws in the rifle look untouched and I sort of have a bad reputation for screwing up screws
By the way, nice looking (seems well used) 12 bore!
Red Leg and Wheels: thank you as well for all the info! The rifle has indeed a butterfly safety, I didn't know it was called a butterfly safety, but seeing how it operates it makes perfect sense. I can also tell you the barrel is definately an FN original barrel as the serial number engraved on the barrel is in the same font as on the receiver which has the big FN logo on it. Underneath the bolt handle you can also find the serial number, and of course they are all matching (luckily).
One Day: no costs for paperwork in Belgium, at least for bolt action rifles, lever-action rifles, blackpowder rifles, blackpowder handguns and .22 pistols. At least if you have a sport shoorters license (which I have), so buying a rifle is just as easy for me as buying a new phone (of course getting the sport shooters license requires some effort). Centerfire handguns and semi-auto rifles require some more paperwork and a couple of months patience however. You also mentioned the position of the base plate lock, it is indeed outside (in front) of the trigger guard. If FN didn't make a factory offering of the 404 at that time this one may indeed very well be a custom shop rifle produced on the request of a customer.
The FN factory in Herstal is actually a one hour drive for me, never visited it however (shame on me, I know) and what's more, as a Belgian I only own one Belgian firearm, and it's this one. All my other firearms are made in USA, Germany, Austria, Czech Republic and Italy. FN nowadays is actually not too keen on selling firearms to their fellow Belgians, an FN SCAR rifle for example has to be imported from the USA if you want to buy one here in Belgium while it has been produced basically around the corner, so it will end up costing twice what its worth, so a lot of guys (me included) buy foreign firearms as they are in fact easier to obtain, at least for the new firearms, the used market is a different matter of course. But Belgium is a weird little country so somehow it makes sense that FN does things like that
No hunting trips are planned so far, in Belgium this also requires a big effort to got the right license for it and time is not on my side. But owning and shooting the rifles that are used for safari hunts give me a lot of joy and satisfaction, so that will have to do for now. I am like the guy who buys a sportscar but never actually goes to the racetrack
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