From the olden days version of the phonetic alphabet, Ack was the word used for the letter "A".
This is why Ack Ack is Ack Ack and not Alpha Alpha.
In The Royal Artillery, an "Ack" is an assistant.
For example, an Observation Post Assistant is an OP Ack.
"yea, and I shall smite upon the left hand and smite upon the right hand, and I shall smite 1,000 souls and not think it too many. Now where did my Ack put my mug?"
Coriolanus, sc 4, act 5, "on the ridge of OP"