ACM(O) is an Aircrewman Observer. He was the smart person with a map who always knew where he was and often shared that information with the Pilot. This was as opposed to the ACM(G) who was an Aircrewman Gunner. Gunners started (I believe on the Westland Scout), which could be fitted with the SS.11 Anti-Tank missile system. This was a dustbin sized wire-guided missile with the speed of an arthritic miner, but could deliver a reasonable punch. The gunner had to optically track the target (Ferranti AF120 Sight), squeeze the trigger, and then when all the smoke had cleared, try to fly the dustbin full of HE to the same grid square as the target. One hand tracking the target, one hand flying the missile, those Gunners certainly had some awesome hand-eye coordination.
All this changed with the introduction of the 2 pilot system, after someone realized that the entire armed action capability of Army Aviation relied on a small number of well-motivated Junior NCO's in the left seat!