OK, here is my take, FWIW. First, check the slide number again; it looks like 803xxx which would be consistent with a Colt number; 903xxx would be Ithaca and they didn't number the slides.
The left side markings and the P are OK; the right side marking looks like a bad pantograph.
The breech face has been worked on, apparently to work with 9mm or at least look like it might, though the firing pin hole has not been bushed (as would be needed for the smaller 9mm firing pin) and the ejector and extractor holes look like standard .45.
I have no idea what the intent was, but I suspect someone was going to try to make a Colt "experimental" pistol and this is what remains. I can almost hear the spiel, "Super secret pistol made for the OSS...only two made...Patton was given the other one...blah...blah...ten thousand is cheap...blah...blah."