This map shows the numbering of the main squares of the map and the lettering of the smaller squares. From the example given in square 116 you will be able to work out where 'R' is in each of the other squares.
However, to identify which 'R' the FOO was referring to you need to know the number of the main square. Is one quoted directly before the 'R'? Or can his note be read as the continuation of a previous entry where the main square's number is given?
I hope that is of some help