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 M11L version of these booklets appeared in late March 2011.
This is the 2nd class from books of 12 with security print ROYAL MTIL, M11L
SG Catalogue/Norvic cross-reference table - click here. See other blog entries for the history of the development of security Machins.