The 78p, 88p, £1.88 and 1st Signed For all have MA13 (although it took me a while to distinguish what was on the 88p which seemed quite indistinct or maybe I need new glasses).
I can detect no obvious code on the 1st Large Signed For.
What is new (to me, at any rate) is what looks like two types of coating on the regionals - thin vertical lines in about the width of the phosphor bands and a cross-hatched band next to each. the cross-hatched area is narrower than the vertical lined area.
This came to my attention as I was about to write how the English had dancing 8s in 88 for some reason best known to someone but the Welsh, Irish and Scots just had side-by-side numerals. the Scots, however, have the luxury of silver numerals whereas the rest of us have white (or whatever colour the paper happens to be). The cross-hatching is clearly evident as a pattern on one of the Scots 8s at some angles. In the the photo above I have just managed to catch the different coatings on the left.