I think Hibc may have hit in the answer with his idea of barreled actions being sold to multiple wholesalers.
If I were a betting man, I'd say FN made these things by the car load, unmarked, so they could be roll stamped with whomsoever wanted their name on it, be that Sears/Ted Williams, Western Field, or Pierre's Sporting Goods in Timbuktu. This would allow caliber markings on the barrel in English or metric units as required.
Whether that roll stamp was to be applied at the factory or by the importer is anybodies guess, though I suspect it was a factory offered service to the various wholesalers.
ST, I bet your dad managed to snag one that slipped through the cracks and avoided a barrel roll stamp.
BTW, highly unlikely you hurt anything with dissasembly, these guns were built like tanks and in all likelihood, your grandkids will be fitting over it when we're all long gone.