Please post a couple of pictures. I think you'll find that there is a thread inside the retainer cover (aluminium base of the upper part of the starter, see figure 16, right-hand illustration). It isn't externally visible, you have to know it's there. It is a right-hand thread. You can unscrew it by tapping anticlockwise with a hammer, using a piece of wood as a punch, on one of the 4 splines. Be gentle.
