They have used various prefixes over the years. I have an "A" prefixed 700 ADL sporter rifle produced in 1979 and an "S" prefixed 700 SS 5-R Milspec rifle produced in 2010. Interestingly, both of the seven digit serial numbers starts with "6". I don't know if that is a coincidence or not.
One can call 1-800-243-9700 with a Remington firearm model and serial number to get the year of manufacture.