Back in the late 1960s the British Wilding stamps that had been issued since the beginning of Queen Elizabeths' reign began to be replaced with the new design that was created by Arnold Machin featuring a side view of the queen's image from a clay bust he had sculptured, this effigy was also used on coins for Rhodesia, Canada and New Zealand.https://en.m.wikipedia.org/wiki/Arnold_Machin
On the 5th of June 1967 a replacement took place of the 1/- Wilding value and was superseded with the new 1/- Machin design, but for some reason on the 28th of June 1967 (just 3 weeks later !) the GPO issued a new 1/- Wilding stamp with 9½mm violet phosphor bands, in essence, it would seem they put the cart before the horse.
Under normal circumstances when a new design of stamp is released such as the 1/- Arnold Machin one in 1967, it is common practice that they replace the previous older existing ones such as the Wilding design, however, this does not seem to have been the case with this particular value and for some reason the reverse seems to have occurred.
I think that this is probably the only stamp that was issued after it had been replaced. WM