The gun was certainly in military service at one time, since it is a military contract gun. And it could very well have been rusted and pitted in service, especially if it had been in the trenches in WWI.
I was not clear. The gun was a GI gun. However there is no reason to believe that it saw combat or was deployed in a combat area without further information. A 1918 gun.
The amount of pitting on the slide is extensive. It also means that the rust itslef was more extensive than the pitting which occurs where the rust is it's worst and has time to eat in extensively. This does not happen overnight. It looks like the type that comes from a gun sitting for months (or longer depending on climate) in a damp holster.
It would have been extremely unusual for any GI or officer to allow their sidearm to rust so extensively. It would have been at least regularly wiped down and removed from a holster particularly in damp or wet environs. That amount of rust does not come from hard use. It comes from neglect.
It could be possible that while in service to be allowed to rust so extensively but it's kinda unlikely.
An arsenal rebuild could have mixed the frame and original slide.
Look closely at the frame and slide. You can see that there was extensive polishing of the slide to remove the rust. So much that many of the roll marks are hard to read. This was done prior to the park job.
The military usually did not parkerize such extensively damaged slides. They did not have to take the time. They had spare slides. So unlikely the military did the polish and park.
So to me two unlikely things.
That's why I think it's
more likely that the gun was a bring back. A gun out of service in private hands that was left to rust and then polished and parked while in the hands of a private owner.
The rust hurt it's collector value but what really hurt was the the polish and park. My opinion for what it's worth.
tipoc