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I'm thinking its not the original cap. HD didn't start using DOT4 until a few years later when they sold ABS bikes.
I'm guessing here, but maybe you can test a little bit of it with water? DOT4 will absorb water but DOT5 will not. Or you could try putting it on a painted surface; DOT5 won't damage paint. I don't know how long it would take for these tests ...
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