If you go to
http://www.zf.com/, and click on "Facts & Figures", and then "Chronicle". You can get a year by year history of the company and their technology.
For 1961's technology, it says:
"ZF dares to enter the world of Formula-one racing together with Lotus Cars Ltd. and the legendary racing driver Jim Clark. The racing cars are equipped with the ZF transmission 5 DS 10, a five-speed transmission with ZF lock-synchronizer mechanism.
Beginning in 1967, Jim Clark drives the Lotus with the Ford V8 engine and the ZF 5 DS 12 transmission"
The page also has a picture of a "Hanomag-Henschel light-duty transporter equipped with ZF 4 DS 10 Synchroma transmission and ZF Gemmer steering type 7328".
So the first vehicle to use the "ZF x DS xx" type transaxle was probably a truck, the first race car to use it was probably the Lotus 21.
The page also has a cut-away drawing of the 4 DS 10.
The site doesn't even mention the GT40.
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According to their "Early History" page, ZF was a spin-off from the early efforts to build a lighter-than-airship.
"In 1915 the ”Zahnradfabrik” is called into being. The precision gearwheels and transmissions produced on the basis of a new patent promise optimal power transmission between the engines and propellers of the Zeppelin airships. Today ZF Friedrichshafen AG is the largest of the companies that emerged from the Zeppelin legacy"
http://babelfish.altavista.com translates ”Zahnradfabrik” to "Gear wheel factory"