Why there were no MA10 or later 2009 50p Security Stamps

Collectors will recall that the security stamps were originally issued in 2009 with no specific year code - the word MAIL appearing where later there was a numeric code.  Whilst most of the NVI stamps - 2nd, 1st, 2nd Large, 1st Large - were subsequently reprinted every year with relevant year codes, the Signed For pair only occurred with MA10 codes, and the £1 ruby didn't appear until 2011 (M11L).  The £2 arrived in 2013 (MA13) - but for the others we will only know if they were even printed when actual examples are found.

But a recent discovery suggests that the 50p light grey (replaced this year with the slate-grey version) won't be included - because a printing of 07/08/09 has now made an appearance.  If it has taken that long to use the original printings of 08/12/08 and 18/03/09, it seems unlikely that there were more printings in 2010 and subsequent years.

In certain light there appear to be slight differences in the shade between the 18/03/09 and 07/08/09 printings:

The only difference I can see is that the bottom margin is deeper on the later printing, but this may not be a constant variation.

As seen on the cylinder blocks, there are 6 grid positions on the new printing, as there were on the earlier ones.

Jim has sent this comparative picture of the 18/03/09 printing with the earlier 08/12/08 printing.  As you can see the later one is much darker.