Sounds like you have a .45 Hand Ejector U.S.Army model 1917. Caliber .45 ACP (or .45 auto rim). They were produced between 1917 and 1946. They were made as a additional side arm for the military, when the factories producing the .45 auto couldn't produce enough to meet the demand. Many were sent to the British for their defense.
It was also produced in .455 caliber for the British and Canadians. Commercial models were available, but by far the greatest number were the military version. Does it still have the lanyard ring on the gun butt? It used either half moon or full moon clips to hold the rounds being loaded.
It would look similar to this one (which is a Brazilian model from 1946).