When the stainless guns first were produced (stainless guns started at CR10001), "Professional Model" was engraved on any gun, including the blue Mark IV, that was fitted with the "Combat Selector" sights. Those sights had the changeable blades for the rear sight. All Mark VI guns had that sight, but after a time the company stopped engraving "Professional Model" on the slide. Your serial number places the gun in the early period of the Detonics stainless production.