Which .45 cartridge do you mean?
Since you say it hasn't any hole for a lanyard ring, it is probably not a Model 1917 in .45 ACP.
So it is a civilian New Service and Colt made those in many calibers, to include the .45 Colt (Long Colt and let us not get started in that old argument
What I don't know is the choices in barrel lengths. We can both hope that somebody more informed will be telling us.
Ditto the nickel finish. If the gun has been refinished you may be able to tell by seeing that the markings are not as sharp as original. How is the horsie on the left side?