This is a new speciality range of Machins. In 2009 the Royal Mail decided to add Security features to their stamps. Originally this was only going to be high value, but then they added them to the various 1st and 2nd class issues as well recognising that these were the biggest volume sellers and more likely to be forged (it was also to stop re-selling of used but un-postmarked stamps on auction sites). In the the following period of time they have changed these security printings several times and in 2010 even added the date "10" into it.
This has already become a serious area of specialist collecting and because of the changes and other factors some are already difficult to get hold of.
UPDATE: For 2011 Royal Mail have replaced "MA10 with "M11L"
We have now published an album for Security Machins which can be found HERE.