You are correct on the date. A large majority of the military Model 1917 S&Ws were shipped in 1918. It is generally assumed that the WWI military M1917 revolvers stopped at serial number 169959. .45 ACP cartridges for the M1917 during WWI were usually packed in three-round clips (half-moon) which are not too often seen. You can get several types of full-moon clips holding six rounds today. These aid in loading and extraction but are actually not needed if you don't mind removing the fired cases individually.

Colt also supplied the US military a very similar revolver, also called the Model 1917. There are some very good WWI reproduction flap holsters available inexpensively. Original holsters are fairly costly.

The condition of yours is very good. Don't mess with it.