There was a period where only a few vehicles were kept at armouries and the bulk were pooled at specific bases as pool vehicles for militia units to draw on for weekend and summer training. The markings I'm familiar with were orange circles with a big black P on them. I have heard of some getting yellow markings with the militia district number on it and yours seems to be M over 22 so it could be Militia District 22.
This is wild ass speculation, so I expect to be corrected or challenged at some point! Good luck with your research.
Cheers! Mike
