R stands for reverse. F stands for Ford, which uses the R designation today for reversed posts.
Ford, starting in the 50's, decided to have their own special morphydite batteries(the 29nF for Thunderchickens as an example) for their vehicles and eventually added the "logic" to standards like 24's and 27's.
The idiocy continues to this day, with most manufacturers building specialty battery parts for each individual vehicle, even though the vehicle doesn't know the difference.
