The early US&S pistols that have no P proof mark on the frame should not have a P mark on the slide either, to be correct. In the mid serial number range, they began putting the P proof mark on the side of the frame and also on the top of the slide just forward of the rear sight. The P will be on the very edge of the top of the slide. In the later serial number range, they still had the P on the frame and the slide P proof was moved to the center on the slide, still just forward of the rear sight.

You should have a copy of Clawson's Collector's Guide to Colt .45 Service Pistols. It explains all you need to know about the pistols in question. You can go to http://www.coolgunsite.com and order a copy for $40.00
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