Technical information: the variables are in a look-up table which includes the variable RFCAREA. This corresponds to URAREA7 (LS member's area of usual residence) in ME71 (LS members present at the 1971 Census). If you rename each of these two variables with the same name (e.g. AREA), the two datasets can be merged. Two of the cases (sampled urban areas) in the Royal Free look-up table had no LS member residing in them at the 1971 Census and can therefore be dropped.