I am following your thread with interest as I am also planning to add AC to my 230 as well. If I understand you correctly, you will drive the compressor with the water pump pulley. Is that what the double groove pulley was used for?
On the W108, the AC as well as the power steering is driven direct of the crank pulleys. I am still wondering what the second groove in the water pump pulley is typically used for?
Do you think or know that the water pump is strong enough to drive the compressor in the long run?
The MB EPC (W108) shows 3 numbers for the pulley. The picture shows only a single groove pulley though.
A 11 420 500 10 A 11 420 501 10 A 11 420 502 10
Possibly one of them is the double groove, in case you can not get a used one.