The New Imperial primary chain case is quite distinctive. The dome part most likely encloses the clutch and the top part would enclose a drive to the magdyno. Probably the larger engine sized bikes had bigger clutches, so the dome piece is bigger.
I also read somewhere that the gear box was integral with the crankcase, quite a feature at the time, when they usually had separate gear boxes, often made by Burmans where my dad worked for many years.