Looks to me like a pair of bolts, probably about 1/4″ , would go through those two slots in the inner cover and secure it to the two matching bolt holes on the side of the countershaft frame. Possibly they are long bolts, or there are spacers that they screw into to hold the cover away from the countershaft frame. Or long studs with nuts either end.
There is also a long stud that goes into that lug on the bottom of the outer cover to the countershaft frame, on the later models,which you may have the same. And it looks like some kind of three tricky fasteners that screw into the outer cover and engage with the three half-round cutouts on the inner cover.
I would mount the motor in position on its platform, and mount the countershaft in position on its swinging H frame and then hold the inner cover up into position and look for what lines up with those two mounting slots in the inner cover. Should be more obvious what goes where when you have all those other bits in place to locate it.