Stainless blades were stamped in that period. According to the Randall Timeline (from Blue Star Knives), the first stamp was “SS” (from around ’63 to ’64), second a low “S” (from around ’65 to ’66), and then a separate “S” (from late ’66 to ’72), and ever since that time the “S” has been integral with the logo.
In Randall Lore #1, however, Rhett mentions that Gary Randall told him, "…on the very first ones, we used two S's (SS), but that did not last long. We used the single low "S" method less than a year and then moved the "S" up by the Randall logo. Later on (about mid 1960's) we had stamps made with an "S" beside the logo as you know it today".”]
Which stamp does your blade have? A knife with low-S would probably go well in excess of $2000 on ebay (more if in great shape or good war provenance). I saw an ebay auction of a low-S #14 with filled hole in brown handle and rivet sheath (as-new except for some verdigris) that appeared to sell in the $10,000 area! Right now Perry is offering a tenite-handled #15 (non-stainless) for over $5000 (Spaceport Cutlery).