CSIs allways had disks on all four. As did the 3.0CS and the 2.5CS.
The only E9 with rear drums is the 2800CS (68-71).
So you either have a 2800CS that a 3.0CSi eninge was transplanted into (without upgrading the rest) or someone has go to quite a lot of length to put a 2800 rear axle and brakes into a CSi (not very likely).