So, I went and looked at my copy of De Geyhn, and the weapon is carried on the left shoulder, which is where you need to carry it if you are going to go through the drill motions. This pattern carries right through to today, basically. The default assumption is for right handedness, and so the locks on all weapons are on the right side.
That said, I have a copy of Sennewald's book on the artist Pieter Snayers who painted all sorts of battle and camp scene paintings, and occasionally guys standing or walking around in camp settings have their weapon on the right shoulder. But mostly on the left shoulder.