After a little research I was able to determine that your pistol was most likely shipped from the Colt factory on November 19, 1918 as part of an 11,000 gun lot with serial numbers in the 521000 to 538,000 range.
Because of pressures caused by the war effort, pistols were no longer manufactured nor shipped in numerical order, and the usual practice of sending production to Springfield Armory in Massachusetts was suspended, and instead this shipment went directly to the Ordnance Depot at Bush Terminal in Brooklyn, NY to be placed on a freighter, and on its way to France, post haste.
There is a thin possibility that it was in a small 500 gun shipment that left the factory on November 25, 1918 that went to: Ordnance Officer, Depot, Ordnance, Washington, DC. That said, it would seem more probable that the later shipment was made up of pistols with serial numbers more toward the upper end of the series.