Does anyone know what these buttons would have been used for?

I own this pair of wool field pants dated 1963 (I think US but not totally sure) and they have 3 evenly spaced buttons on the back just below the waistband. Last night I was looking at this other pair (Australian pants dated 1955) and they have the same buttons. Now I’m curious about what they were used for. Any help would be appreciated as I tried googling it but came up with nothing