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.
This stamp is from the Arnold Machin Centenary Sheet and comes with top or bottom or cover margins - your preference if available.
The code is ALAM11ROYALMMIL, i.e. a year code of AM11 and a source code of MMIL
SG Catalogue/Norvic cross-reference table - click here. See other blog entries for the history of the development of security Machins.