Join Date: Sep 2008
Location: Bavaria, Germany - the center of 15th and 16th century gunmaking
They seems strange indeed and troubled me too at first; somehow 'out of period' (or rather of what we expect to be 'period').
On the other hand, quadratic bolt heads were in use since at least the 15th c. as a small Gothic alcove gun, preserved in virtually 'untouched condition' in my collection, shows.
Similar devices are depicted in Philip Mönch's Kriegsbuch (Book of War Techniqes), 1496, fol. 32r.