9 Mx. Would certainly be the abbreviation for 9th Battalion The Middlesex Regiment (DCO).

They were a Territorial Army Battalion Head-quartered at Willesden in what is now North West London. Sometime (between the wars IIRC), they were converted to Royal Artillery but kept their Middlesex Regt. title. Later the unit at Willesden became S Battery, 562 Parachute Light Battery Royal Horse Artillery (armed with 4.2 inch mortars) until disbanded in the 1960s.