It appears there are two different size belts available for the 81 240D (970 and 1000 mm), my guess is that you have the incorrect belt:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42EU1DEABM&year=1981&make=MB&model=240-D-002&category=All&part=Accessory+Drive+Belt&appTransmissionType=Automatic&appEngines=_any