>>>>> Andreas Schwab wrote:

> The score rule for extra headers is a five element element list (the
> fifth being the actual extra header to match against), unlike the other
> string keys.  But the customize type for the "Extra" entry is the same
> (gnus-score-string) as for the other keys, thus it will not match, and
> you cannot enter such a score entry via customize, and any existing
> entry is presented as a lisp expression.

I've installed a fix in the Gnus trunk and the v5-10 branch.
Here's a patch for No Gnus 0.6 that you use: