It is very frustrating not being able to find literature for the MG9 grinder, but I'm slowly coming to an understanding.
So, I saw this photo from the web which is a good rear shot of another machine.

Clearly there is no counter-weight on the MG9. The four holes appear to be motor mount holes which means that the original 3 phase motor must have been inside the base casting. Mine has a large frame single phase motor fitted to a shelf in the cabinet. I believe that the two holes cut in the top of the cabinet are not original (and the access panel in the rear on mine is certainly not), and were put in to facilitate the fitting of the single phase motor (the capacitor pokes up through the larger square hole, the belt passes through the smaller).
It''s very neatly done, and a very good quality motor, just not original fitment. I am undecided as to which way I will go with this. I could fit a 3-phase motor back in the original position and use a VFD, or I could persevere with the single-phase one.
Another question I can't answer is whether the original drive used a single vee pulley or a two-step pulley. My spindle has a 2-step pulley fitted, the motor a single-step pulley.
The three holes in the end of my slide also appear very faintly in the pic above. They seem to have been filled and painted over. I can only assume they were retaining a way cover of some sort.