United Kingdom Special Mail Service
The 30 cent denomination was intended for regular 1/2 ounce letters
and the 50 cent denomination included a registration fee.
During the 1971 British postal strike a premium was applied to allow mail to be sent by air to the Netherlands and then delivered by a private service in Great Britain. The strike ended on March 8 which was the day on which the higher value denomination was released.
"Cook Islands." Stanley Gibbons Stamps of the World. 2002.
Kloetzel, James E. ed.
"Cook Islands." Standard Postage Stamp Catalogue. Sidney, Ohio: Scott Publishing, 2008.