Here is the answer from RCBS:
"We started letter stamping our dies in 1950 and in approximately 1955-1967 we began to date stamp the production dies while continuing to letter stamp Special Order dies. We do have a chart that is not for public use but if you wish to call us and tell us what is stamped on your dies we can tell you the year of manufacture. "
Since RCBS dies are covered by their "lifetime" warranty, why would date of manufacture matter, other than simple curiosity?