Hi,
Sorry, it my fault, I should have realised you are not yet familiar with the terminology.
This isnt gospel, but my understanding is that the Essex Regiment originally had 4 Battalions (Bn.) of Territorial Soldiers (TA)....during the first world war there were many more but some were amalgamated whilst others were disbanded, hence the title 1/3, 1/4, 1/5, 1/6.....also helped to differentiate from the 1st Bn., which was a Regular Army formation.
As war loomed, many TA units were retrained as Anti-aircraft units (normally HAA - Heavy Anti Aircraft, or LAA - Light Anti Aircraft).
This involved their command changing from their parent Regt. (Essex in this case) to the Royal Artillery (RA), although they could then stay with their parent Regt.!!
Upon war being declared the TA were amongst the first called up, being trained already.
Pre-war though, the TA were pretty underfunded and generally ill-equipped (sound familiar, guess nothings changed!!), so their numbers were not up to the size of a normal Battalion.
As a result, men who were conscripted / drafted after war broke out, could be posted to a unit that appeared to be a TA unit.
As you say he was conscripted he doesnt sound like a pre-war TA.
The Essex had a further Bn. (9th) converted to Royal Artillery, but I think he was probably 1/6th Bn.
His anti-aircraft Regt. would have consisted of 3 Batteries of guns, each battery having 3 AA guns, so probably 9 guns in total.
If you contact the Essex Regt. museum, they may be able to give you more help. If you are lucky, they will have copies of the war diary for the 1/6th, this will tell you exactly where they were and when, using this you can plot his movements fairly reliably.
You could also obtain his service history from the army, which gives you his postings etc. it is interesting, but there is a looooong wait for them.
I hope this helps, feel free to ask any more questions if you need to.
Regards
Pete