Here are some more Post And Gos to look out for. The Flag is now in the new font 4, and also has the BPMA overprint, both appearing on MA13 backgrounds.
I have also been fortunate to get hold of the wonderful 1st Class Large error (on undated background) on a Stampex 2014 strip. This is well worth looking for! Of course, on a pictorial that would have been obvious but on the wide space available on the Machin head type it looks OK at first glance.
There is also a BPMA strip on MA13. These dates in the background will make collecting expensive as there seem to be two versions of each type at the moment - one on undated background and another on a background with a year code. So how long will it be before we get MA14 too for some issues and there is always the possibility of old rolls in future being put in so MA13 or MA14 might well appear unexpectedly any time!