Your Colt was made during World War One from 1917 to 1918.
General Pershing wanted every actual combat soldier to be armed with a pistol for use in close range trench warfare, but the supply of 1911 automatics was too small to meet the demand.
So the government contracted with Colt and S&W to produce revolvers chambered for the standard .45 automatic cartridge.
To make it possible to shoot and eject the rimless auto cartridge, the guns were set up to use "half-moon clips". Each clip help three rounds, and to load, the user just dropped two clips into the cylinder.
The clips also made it possible to eject the rounds.
S&W and Colt built these Model 1917 revolvers on their larger frame double action revolvers.
S&W used their "N" frame, and Colt used their New Service model.
Government serial numbers were stamped on the gun's butt, and lanyard rings were installed.
S&W used only the government serial number, but Colt decided the Model 1917 was just a continuation of the New Service, so they also stamped Colt New Service range serial numbers on the frame and cylinder crane.
This causes some debate among collectors as to just WHICH serial number on the Colt is the "real" number. The government considered the number on the butt as the "official" serial number and used it. Many collectors use the Colt number.
Early Colt Model 1917 revolvers had cylinders with "straight through" chambers, and these early guns cannot be fired without the clips.
Later guns had a "step" in the chambers which allowed the gun to be fired without clips, but the gun cannot eject the cases. To remove the fired cases a pencil or dowel must be used to push them out.
The cartridge makers used to make a special cartridge for these Model 1917 revolvers, known as the ".45 Auto Rim". This was a .45 ACP cartridge made with a rim that allowed firring AND ejecting cartridges without the clips.
After WWI, the Model 1917's were stored, with a large quantity of Colt models being given to the US Post Office for use in guarding the mails, after the 1930's bandits started robbing the US Mails.
When World War Two began, we were again short of pistols, so the Model 1917's were taken from storage and re-issued.
Work guns were re-finished with a parkerized coat instead of the original brushed blue finish.
After WWII, the Model 1917's were declared surplus, and large numbers were sold to NRA members through the Director of Civilian Marksmanship (DCM) program for around $25.00.
In the 1960's, the US Post Office sold their Model 1917 Colt's on the commercial market.
Your particular Colt Model 1917 appears to have been made in 1918.
If in original condition, it should have a brushed blue finish, and smooth walnut grips.
As long as it's in good condition and is in proper time and adjustment, it's safe to fire with standard .45ACP ammunition.