Is it possibly hard chromed? A lot of folks who bought the black 457 pistols with the cheap Sharpie marker finish, had them hard chromed once the finish wore off.
One way to tell is if your slide has roll marks. IIRC all the 457S guns had the cheesy laser etched markings.
No matter, a 457 is a great carry gun. I carry and shoot mine regularly. A lot of gun for the money and head and shoulders above anything they have made in almost 20 years - as far as quality and shootability.
Regards 18DAI