According to:
Merionethshire car registration letter code FF (oldclassiccar.co.uk)
Which, I hasten to add, is not infallible, the last "naturally occurring" FF registration before year letters were introduced, was HFF in 1963, which would indicate that the subsequent letters (JFF, KFF, LFF etc.) were issued by DVLA Swansea as "Age Related" plates, probably in the 'nineties.
Merionethshire car registration letter code FF (oldclassiccar.co.uk)
Which, I hasten to add, is not infallible, the last "naturally occurring" FF registration before year letters were introduced, was HFF in 1963, which would indicate that the subsequent letters (JFF, KFF, LFF etc.) were issued by DVLA Swansea as "Age Related" plates, probably in the 'nineties.