That looks like a British 2nd Model HE ( forget the nomenclature) .455. I had one that had been converted to .45 ACP/Auto Rim and was marked ".45 AP". Some, including mine, were not caliber marked by the factory. The barrel looks longer than a 1917.
If there's no serial number on the rear face of the cylinder it's been "shaved" and it's a British service revolver converted to 45 ACP and Auto Rim. But they are usually only stamped 45 AR because 45 ACP isn't considered safe to shoot in these non heat treated cylinders.
A 1917 always has the left side of the barrel stamped 45 CAL. If it doesn't and I don't think it can be with that length barrel, it has to be a 455 BSR. Even though it has 1917 grips.