The marks that cannot be removed or altered are the manufacturer name and address, country of origin, and the serial number. These are all on the frame, the recognized "firearm". The caliber is also supposed to be marked on the chamber.
As far as your project, go ahead and buy a new series 70 repro, but sell the slide and replace it with a new Caspian barstock slide. You'll end up with a set that's easier to fit frame/slide on, and a clean slate for your engraver. No slide width lost, no typical Colt to Colt fitting issues (Some of the newer 70's have had as much as .016"

lateral play. Way too far out to squeeze, welding is a must.)
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