The "S" shaped iron fastened to the wall is the end cap for iron/steel rods that run across the building to keep the walls from bowing out. There should be s corresponding one at the same location on the opposite side of the mill.
I'd wondered about that myself. If it is, the builders were well traveled. I'm finding the same fixtures on building 1400 miles from there.
is it possible that the S mark might be a makers mark??
That's a beautiful old building. I've seen that same "S" shaped iron fitting on quite a few of the older brick buildings I've been photographing too. Is that something structural? I'm afraid I don't know a great deal about construction.