I am looking up 3 ATA personnel who were R.A.F. pilots seconded to A.T.A. they were all of 'Ancient and Tattered age' but appear to be pre-war pilots

Their AIR 78 records were all stamped Militia but there is an additional code one is DU another KD and finally JH
does anyone know what these two letter codes mean?

I find it interesting that they were probably Sgts or Flt Sgts or even WO in RAF (i.e non commissioned) but because they were in ATA they could progress to First Officer or maybe higher..