I bought a couple long metal tubes at the TO VW show this summer for
nearly free.

I figured, due to the length, they must be for a Vanagon:-)

There are two tubes, only slightly different.
They are metal, over an inch in diameter, over 4' long.

I found a tag on one with a possible part number:

251 121 399B

Any idea what it is?

Thanks,
...Markus