HC Deb 06 December 1968 vol 774 c584W
Sir S. Summers

asked the Postmaster-General if he is satisfied that letters stamped with a 4d. stamp and a Id. stamp in practice receive the service of first class mail, in view of the fact that the methods used in mechanical sorting only recognise a 5d. stamp; and if he will make a statement.

Mr. Stonehouse

Yes. Letters bearing a stamp, or any combination of stamps to the value of 5d. or over are directed by the machines into the first-class stream and letters bearing a 4d. stamp into the second-class stream. This is done by detecting the phosphorescent marks on the stamps.