Yes, it looks just like an old school radiator cap.
You can see it in the Bentley page 19.8 in the waterboxer cooling.
Page 19.4 in the diesel cooling, same cap.

Mark

J Stewart wrote:
> Really? Thats good to know. It looked just like any radiator cap you'd
> see on a piece of Detroit iron, which led me to think it was wrong. I
> hope these guys get it fixed and don't end up trashing it.
>
> Jeff Stewart
>
> ------------------------------------------------------------------------
>
>     The metal coolant tank cap is stock for 82 and 83 diesels. Very early
>     waterboxers too, since they used the 83 diesel tank at first.
>
>