About all I can tell you is what you already know, that it is pre-WWII, probably made in 1939 or 1940. (The German invasion of Belgium was in May, 1940; one source says the Germans started serial numbering the HPs at 50,001, so there was probably some gap to prevent overlap.) Barring any military markings, you can probably assume that it was a civilian pistol.
The whole BHP serial number story is confusing. If a government ordered, say, 5000 pistols, and wanted their own series, FN was happy to sell them numbers 1-5001, never mind that some or all of that range had been used before, perhaps more than once.
The sight replacement, if it can't be reversed, ruins the collector value of the gun.