I'm a little embarrassed to admit this, as I changed the tumblers and steering wheel lock assemblies in both my SDL and 300D, as well the part pictured. But I can't remember which part has the spring in it. My best guess is that it's not this but the steering wheel lock.
BTW, these guys claim to have the whole assembly in stock:
http://www.new-part.com/product/mercedes-w123-w126-300-steering-lock-housing-oem