As previously mentioned, the Colt M1917 was a variation of Colt's New Service Model revolver. Several variations of the New Service Model were manufactured between 1898-1944, in many chamberings, but the 1917 was only chambered for 45 ACP. 45 Auto Rim ammunition will also function in these guns, and was in fact designed specifically for them.
Because 45 ACP is a rimless cartridge (meaning its rim is the same diameter as its case body), typical double action ejectors (which the 1917 has) won't remove cartridges that are loaded into it individually. A "half moon clip" was devised, which is a simple piece of spring steel that holds three 45 ACP cartridges and lies atop the ejector, allowing the easy removal of all cartridges at once.
Also necessitating the use of the half moon clip is the fact that early Colt M1917s did not have a "step" machined into each chamber for the case mouth to rest ("headspace") against. This left only the clip to support the cartridges against the strike of the hammer. Thus, some half moon clips became deformed and unusable after some use. Later guns did include a headspacing step inside each chamber, as does the gun pictured.
There is also a "full moon" clip, which holds six cartridges arranged in a circle.
Trouble related to the use of clips was the main impetus behind the 1920 introduction of the 45 Auto Rim cartridge, which is simply a rimmed cartridge that is in other ways identical to 45 ACP. It may be used in any of these revolvers, with no worries about headspacing or extraction. That said, half moon clips can be a pain, but they're not as awful as some would have you believe.