RIA/EB is for Elmer Bjerke who was foreman of small arms inspections at Rock Island Arsenal from 1947 to 1958. The rifle was overhauled at some time during those years. That is probably when it received it's slathering of cosmoline.
Just curious, where did you spot the "EB"?
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The 8th photo in post #5. There is a boxed stamp with RIA over EB. The B is faint, but can be seen, and the only other stamp like that was RIA over FK, and it's not a K.