The 45 acp has no rim so it won't fit and it will turn this antique in a hand grenade! The barrels don't have any Belgian nitro proof marks so I would treat it as a black powder pistol just to be safe.
Please just take it to a gun smith and ask him to slug the barrel so you get to know the exact calibre. Don't start experimenting, you'll end up hurting yourself or the friend who will be standing next to you.