ACH was probably "Aircraft Hand", a term used for general duties.
Ball is probably "Balloon". So ACH/Ball was probably the classification for someone working with barrage balloons.
ACH is shown here:
http://www.associations.rafinfo.org.uk/acronyms.htm
A reference here to when the ACH term was abandoned post-war:
https://www.flightglobal.com/FlightP...20-%201738.PDF