Just a guess, but looks like it could be a fuel pressure valve of an older vintage.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1VX0OZ4D924S15TCR8&year=1985&make=MB&model=500-SEL-001&category=All&part=Fuel+Pressure+Regulator

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000