Brake pad dampers are required to reduce squealing of the brakes by properly positioning the brake pad in the caliper.

There's a variety of dampers, and this table may help identifying which is which.:

Type Year used for part (20) size part (21) size  
86.5 - 88 front 928.351.096.19 36mm 928.351.096.18 42mm  
- 88 rear caliper 928.352.421/422.01 928.352.096.11 28mm      
rear 928.352.096.10 30mm      
87- 89 front caliper 928.351.421/422.02 965.351.096.00 44mm 964.351.096.01 36mm  
front caliper 928.351.421/422.02 928.352.096.15 44mm 928.351.096.16 36mm  
front caliper 928.351.421/422.01 928.351.096.17 44mm 928.351.096.19 36mm  
front caliper 928.351.421/422.00 928.351.096.18 44mm 928.351.096.19 36mm  
rear caliper 928.352.421/422.01 928.352.096.01 28mm 928.352.096.11 28mm  
rear 928.352.096.00 30mm 928.352.096.10 30mm  
89 - 95 front caliper 928.351.421/422.02 928.351.096.15 44mm 928.351.096.16 36mm  
front caliper 928.351.421/422.01 928.351.096.17 44mm 928.351.096.19 36mm  
front caliper 928.351.421/422.02 928.351.096.18 42mm 928.351.096.19 36mm  
front 965.351.096.00 44mm 964.351.096.01 36mm  
rear 964.352.096.00 30mm 964.352.096.01 28mm  
             

Brake pad damper example

ps: I'm not 100% sure how accurate this really is. Any updates appreciated!