This Colt M1911A1 was produced in 1941 and was near the start of when Colt started to utilized parkerizing in their production finishing. Colt began parkerizing pistols in May of 1941 at about serial number 734000 and shortly after was phased into normal production. At roughly 760000, bluing was completely phased out and parkerizing became the standard finish.
The RS Inspection mark is for Colonel Robert Sears. Col. Sears was the the Army Inspector of Ordnance at the Hartford Ordnance District office from August 3rd 1937 - June 30th, 1941. His inspection mark is seen on pistols in the 723000-750500 range.