As to the guns that were to be registered, what makes a hunting rifle a hunting rifle? It's a question as old as gun control. The turn-bolt action rifle was patented in 1895 by Paul Mauser and was the prototype for every military service up to WWII when the US introduced the M1 Garand semi-automatic rifle. A gorgeously engraved, gold-inlaid square-bridge Mauser with handsomely carved black walnut stock is still based on a military action, the German service rifle of the day. Would it be subject to registration? No clue here.
The Belgian service rifle of the day as well, specifically the M1935 and (in the reserve units) the 1889, both chambered in 7.65x53mm. The answer that matters in this instance is what the definitions are according to the Belgian weapons law of 1933. A rifle with a Mauser action would have been classed as a "weapon of war" if it was in a "military caliber" (the law was less than clear on whether that meant only Belgian military calibers or those of other country's armed forces as well). If it was chambered for a non-military caliber, it would fall under hunting weapons, and thus not be required to be registered.
According to the story, the gunsmith was an armorer to the king of Belgium. I don't know whether there was just one of those, or whether every gunsmith who happened to sell a gun to the royal family could claim the mantle. If it was an exclusive club, it might be a way to subset the data and get closer to the town. Or he might have been an old braggart.
I'd go with the hypothesis that he'd received a "royal warrant" for supplying one or more weapons to the court. There's currently only one warrant holder for hunting guns, and that's E.J. Binet & Sons in Brussels, but they've only been in business since 1927, so our gunsmith might easily predate that. However, to illustrate wider possible interpretations, Binet doesn't make their own guns, they buy them from a small number of gunmakers, including FN (which is, after all, a Belgian company) but also a few small Belgian workshops (Duarme Liège and Lebeau-Courally); it is possible that our guy worked for one of these companies before setting up his own shop, and that one or more guns he made while working there was sold to the court.
Charley was "well over thirty," according to Dad. That would place his birth year somewhere between 1925 and 1930.
If DeNaer was "well over thirty" in 1955, it would put his date of birth prior to 1925. But not by too much, because if he'd been 18 or over by early 1940, he'd have been drafted, and
that would further have complicated the story.
That said, I'm now open to the possibility that your father's story about Cpl. DeNaer may have been somewhat mutated in being passed on, so that, for all I know, it might not have occurred in 1955 in the first place. Let me make one thing very clear: the story
as it is presented in the link from the OP is implausible. In particular, it this passage that is troublesome
as written:
A year or two later, the blitzkrieg rolled across the Low Countries. One day not long after, the war arrived in Charley's town. A squad of German SS troops banged on the door of a house that Charley knew well.
If that's not an accurate representation of the story Charles DeNaer told your father, and--to use your example--the execution occurred in 1943 or early 1944 rather than 1940, that makes the story a lot more plausible. The "SS troops" might have been
Sicherheitsdienst ("security service"), which was a "civilian" SS component.
Later in the occupation, but before the Allied invasion, folks might have grown restless, perhaps taking a stray shot at the isolated patrol. That might have prompted the Germans to take harsher measures and track down the guns they could find.
Direct armed action started to become more common shortly after the German defeat at Stalingrad, partly because it made it clear that Germany could lose the war, and that realization also prompted the Germans to become repressive in the occupied territories.
However, it's still thin. The Germans simply did not routinely conduct summary executions in the low countries, except in direct reprisal for assassinations of German personnel or prominent collaborators (for example, on 29-Nov-1942, eight people were shot in Charleroi in reprisal for the assassination of the quisling mayor), or acts of major sabotage. Those killed were typically people already in Nazi custody, either as hostages or on suspicion of resistance activity (often already designated
Todeskandidaten, "candidates for death"). Captured resistance members were frequently executed, but not before having been thoroughly interrogated; most were sent to concentration camps. In situations like the one in Cpl. DeNaer's story, the more typical punitive action would have been to ship the whole family off to a concentration camp.
Another thing to bear in mind is that the Germans need not have gained their information from Belgian government records. Collaborators ratted on their neighbors in every occupied country, and the Flemish-speaking part of Belgium in particular had a comparatively high number of them. All it would have taken was one neighbor holding a grudge who remembered from before the war that the father had this old German pistol...