Mike,
Welcome to the Forum. That is a very nice old M1917. However, like John, I also believe that it has been very nicely refinished.
To help you a little in establishing the production date of your revolver, there are three characteristics of the early M1917's that we rely on. The earliest ones had fine concentric grooves on the sides of the hammer. That feature was dropped approximately Feb. 1918 at serial number approx. 25,000. The early guns also have the inspector's initials GHS in a circle on the left side in front of the hammer. This was replaced with the Ordinance Bomb mark after April, 1918, The early grips had the concave top and that was replaced around March, 1918 and serial number 30,000 with the non recessed tops in the later guns.
Since your gun does not have the concentric grooves in the hammer but still does have the GHS initials and the concave grips, we can conclude that it was produced somewhere in the March-April 1918 time frame. However, as always with the S&W factory, things aren't necessarily as they seem. The grooved hammer was replaced somewhere around 25,000 and your's is 18,788 so we could conclude that either the hammer was replaced later in the guns life (perhaps when it was refinished) or the gun was assembled at the factory later from an early frame and grips. More likely, your gun was simply a little out of the normal production sequence and/or the estimates of the changes aren't exact.
Bob