Understand that we have two different things here, or at least two models.
1969 and earlier was a tank to accomodate expansion of the fuel in the main tank; it expanded into the overflow tank via vent lines, and the vapor (or fuel if the expansion was too great) vented to atmosphere via a small vent line from the expansion tank to the atmosphere; you can usually see this little pipe near the exhaust; sometimes it has a cone shaped end. I've cut these open, there's nothing inside but the ends of three lines. Two connect to the vent lines on either side of the filler neck where it enters the tank and the third vents to atmosphere. Nothing else.
1970 saw (as the photo shows) the elimination of allowing fuel and vapor to escape to atmosphere (smog you know) and there was the secondary unit (were they all one?? I can't tell) that had valves and maybe internally, a carbon canister. This kept the system closed.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America
