It is a torx bolt - they are differentiated by inside and outside. The inside torx bolts are more common, but every now and then an outside torx bolt shows up. You can find the socket for it, they're numbered just like the inside torx sockets are, and look like an extension. A lot of older Audi's had outside torx head bolts. Maybe you could find a socket from an Audi dealer?

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick