Strange, do you have something that looks like this in the heater line?

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41O90VH5S3&year=1978&make=MB&model=300-D-002&category=All&part=ACC+Mono+Valve