Machin series - Wikipedia
https://en.wikipedia.org/wiki/Machin_seriesBooks• Muir, Douglas (2007). A Timeless Classic. The evolution of Machin's Icon. The British Postal Museum & Archive and Royal Mail. p. 236. ISBN 978-0-9553569-1-9. With the genesis of the Machin effigy, the author, curator of the BPMA, described the context of philatelic creation in Great Britain during the second half of the 1960s.
List of Machin stamps - Wikipedia
https://en.wikipedia.org/wiki/List_of_Machin_stampsThe decimal Machins have been issued throughout Britain's period of decimalisation, and continue to be produced today. During the 1970s and 1980s, Royal Mail issued 12 Machins in a larger format. Other major variations include: Elliptical perforation: In 1993 Royal Mail started issuing Machins with two large elliptical perforations, one on either side, as a security feature. Some Machins have both standard and el…