At somewhere near #10225 Colt's instructions were to stamp only 4 digits on the barrel and cylinder (not 5 as formerly done). I wonder if this is a "U. S." gun with a tiny "A" under the trigger guard SN, right behind the hammer on backstrap, on the periphery of cylinder, and underside of barrel? The "U. S." to the right of the patent dates may be obscured by rust, or may have been removed. Ainsworth ("A") was the dominant Government Inspector for the early Colt SAA purchases.
Most likely this is a 45 Colt, and you can confirm that by peering through a cylinder chamber.