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I really do not know but according to date codes this is the date, R for November and O for 1977.
See it at: http://www.remingtonsociety.com/rsa/que ... arrelcodes
Model 600 ends on 1967 and Model 660 on 1971, so neither case is right.
An explanation can be that they change the barrel at factory on 1977 or that the rifle was ansambled after the end of productions with remaining components but this is only my humble opinion.
Sorry for not having the answer.
Thanks
Martin