I was researching the Ferrari Testa Rossa the Henry Ford Museum traded for some old American car and then remembered that for several decades they used to have a Mercedes 300SLR. I heard long ago that they got the car in trade for the secret of "floating glass" but thought the Museum was pretty well separated from Ford Motor Co mfg operations so was surprised they would give a multi-million dollar industrial secret away for a mere old car. Also I heard Ford wanted to play with the desmodromic valve system. It would seem that this is just rumor in either case because neither company could use the other firm's idea unless they paid patent rights.
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I also read recently that Mercedes got the car back. Maybe after the problem in England with the W196 they decided to read their contracts more closely and discovered the 300SLR was only on loan to Ford and they could ask for it back.
It might be number 0704. I also heard it was one of the ones with the airbrake (big flap behind the driver) but can't remember because I last saw it oh, 50 years ago. I'd love to hear what they had to pay for it if they had to buy it back.