First line on this diary -- "Purchased horse, cart, and harness for use as an _____ for G.O.C. 3rd Division". The only thing I can get that word to be is "office" with a very small initial "o". Did they mean they purchased it for the office? Or is that some other word?
Yep it certainly looks like office! I am guessing you are right and it's a mobile office for the GOC (general officer commanding). These days they would have one in a motorised vehicle for use in the field in exactly the same way!