Most popular variants of the header value (we only show this when there's just a bunch of variants):

Purely informational and non-standard header initially established in memory of a popular fantasy author Terry Pratchett. Counterintuitively for most IT people, the most frequently seen value GNU is not related to the open-source GNU project, but rather is a sophisticated telegraph code originating from Pratchett's "Discworld" cult series.

Reference...

We have seen 471 websites setting the X-Clacks-Overhead HTTP header and 17 unique values of this header.

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