Well, it's a mix-and-match gun with sight modifications. Hard to put a value on that, but if I saw it on a shelf at a gun store with a $400 price on it, I'd probably buy it. The thing is that original or not, an early .45 ACP with adjustable sights is not something you see every day. At $500 I would negotiate, and at $600 or above I would probably pass it by in silence.
Further info: with that serial number the gun was shipped in early 1916. The crown over 41 (or 11?) is some kind of British viewmark; my converted .455 has a crown over 30 in that location. The serial number on the cylinder suggests it was manufactured in 1938 and originally installed in a Brazilian contract 1917. The rear sight, with two screws in the rear, is a 1930s design. Before that, I believe there was only a single screw on the rear of an adjustable rear sight. The front sight looks home made to me. I don't recognize that as a standard front sight from any company. If you pulled the rear sight, there might be a serial number on the underside of the long base; that might help determine what model that sight was on originally.
If we assume the Brazilian was not available for parting out until the 1950s at the earliest, that dates part of the modification to this gun. But better adjustable sights were available at that time, so the use of the older prewar form may indicate that this gun was originally a target-converted .455, and the addition of the .45 ACP cylinder came along later.
Also, I don't think dropping in a 1917 cylinder would be a simple task. Aren't those cylinders shorter than the regular .44 Special/.455 cylinder? If so, the B/C gap would need to be adjusted, which probably means removing the barrel and planing the back of the surface that mates to the frame to allow another full turn as it was threaded in.
Or maybe my fevered brain is seeing more complexity than needs to be there. Apply Occam's razor. In any event, this is a very interesting gun.