If you look at the pulley on the front of the compressor with the engine running and the AC off, the center section on the pulley is stationary while the outer pulley is spinning. When the clutch engages, the center section spins (assuming yours is like the bottom photo):
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ425F0ZP863&year=1981&make=MB&model=300-D-002&category=R&part=A%2FC+Compressor&appEngines=_any