I have both passenger's side window regulators that include the rail it sounds like you're talking about (if it is the lower rail that the window is attached to).

http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=2CK0P24R12CN0P007T&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1981&product=O3040-16213&application=000022651

E-mail me at vaughanwoodruff(at)gmail.com if you're interested.

V