Chuck -- I'm going by memory here of the obscure M-B re-circ. kit for better head cooling that has been recently installed on my '71 280SL...but yes, I believe you may have spotted that M-B kit in those pulled-engine photos near the bottom of the long list of pictures on the Ebay site.
We're talking about the optional (long-lost?) M-B re-circ line kit for additional head cooling that was recommended by Gernold. And on my '71 280SL we've recently gone beyond this factory kit. As described in a recent posting, we went further by adding a large helper pump (along with a custom long/high-volume arrangement of pipe & hose) where the main coolant line comes out of the left, bottom of the radiator. And everything (ie, the M-B head cooling re-circ. kit plus my new "helper" custom arrangement) does re-enter thru the thermostat, as you've mentioned.
Also, Gernold made a tiny modification to my throttle body, which as I understand improves timing at idle which in-turn also helps hold engine temperature while at idle. As discussed earlier I will not rest until my old (new?) '71 280SL can & does hold engine temperature just like a new car will. In my probable overly-simplistic view it's just a matter of applying the ideas of basic physics.
Tom in Boise
'71 280SL 4-spd, signal red w/lt. tan interior, restored/enhanced
