1: Steering gear ZF531 used from ch. number 20-0001 until 20-09999.                       
 2: Steering gear ZF532 " old" used from ch. number 20-010000 until 20-041711.  
3: Steering gear ZF532 "new" used from ch. number 20-041712 until 20-117901.
The difference between ZF531 and ZF532 "old", is that the top of the case on ZF531 is bolted.