From the 16 September 1968 when a two tier postal system was introduced, there were diffrent postal fees for letters sent by either the first or second class service

     At this point the currency system cahnged from the imperial system, (pounds, shillings and pence), to the decimal system, (pounds and pence).

First Class
Charge                    Weight

    3p                     4 ounce (maximum charge for letters to any destination)

Second Class
Charge                    Weight

    2½p                     4 ounce (minimum charge for letters to any destination)