That battery does sound weak. However, it also seems to be on the verge of starting. When I got my 300D started for the first time in 5 years, it sounded like that, too. I could see fuel at the injector ports, but still I had to crank the starter for up to 20 seconds at a time to get the fuel going. Here you gave up after a few seconds. The manual says it's okay to do so.

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1975 300D