From 17 February 2009 1st and 2nd class Machin Definitives (and some higher values) were be produced as self-adhesive Stamps with enhanced security features including semi-circular slits and an irridescent overprint.
These initiatives are part of the on-going development of security features for Stamps. The slits are designed to inhibit the removal of these Stamps for fraudulent sale and re-use. The overprint features the words 'Royal Mail' in a pattern reminiscent of security features on banknotes.
The 2011 issue (coded M11L) was printed from October 2010 and appeared in January 2011. Many Post Office branches do not stock these as they have sufficient stock of 1st class special stamps, customers wanting definitives are offered booklets or stamps from business sheets.
SG Catalogue/Norvic cross-reference table - click here. See other blog entries for the history of the development of security Machins.