None were sintered. Below is from the company that blued them...
"During the early 1960's, the Winchester Model 94 was redesigned to accommodate a more economical method of production. The re-engineered receivers were machined from a graphitic steel casting. This new material would not accept the normal Du-Lite Oxiblak bluing process, used by Winchester since 1940, without discoloration. The color could vary from copper to plum red, depending upon the individual piece. The chart below shows dates and serial numbers for the three different blackening processes used by Winchester on these receivers between 1964 and 1981.
Mfg. Date Serial Numbers Blackening Process
1964-1968 2,700,000 - 3,185,691 Du-Lite 3-0 Process
1968-1972 3,185,692 - 3,806,499 Black Chrome Plate
1972-1981 3,806,500 - 5,024,957 Win Blue/Oxiblak
(Win Blued receivers were iron plated and blackened with Du-Lite's Oxiblak.)"