Wiki to the rescue …

I am just finishing my first turma of auxiliary cavalry (1st Corps figures). I was puzzled by an item of equipment carried on the rear of each figure’s saddle, and wondered how to paint this.

I could see that one detail was some kind of cup, pot, or ladle. But the item it rested on, which looked like the top of a czapka (a Polish lancer’s cap), was a mystery, until I discovered this on Wikipaedia, a detail from Trajan’s column:

I concluded that the rectangular item on the top left is a loculus, a leather satchel.
Image: Wikimedia commons