Curious stone cannon

[Marcus den toom]

Matchlock, you are right i noticed this too. But when looking better, i wonder if the other side might have a same hole (the picture is taken from the left, but i notice a similiar chisseling at the right side).
Maybe these aren't touch holes but some kind of carrier holes (maybe with sticks to move the barrel around or up and down?)
just trowing something into the discussion.