Greetings cards could be sent at the (cheaper) 'postcard rate', as long as flap was left unsealed, until 15th September 1968. The reason that the flap had to be left unsealed was so that Post Office staff could (theoretically) check that the card contained no more than 5 words "of conventional greeting" (which was the maximum allowed at the lower rate of postage).
On 16th September 1968 the new system of First and Second Class postage was introduced, removing concessions for postcards or greetings cards.
Chris