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…
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.