The xkb spec defines that the 7th element of the DeviceValuator key action description is "valuator 2 value". See https://github.com/freedesktop/xorg-xorgproto/blob/21e05da99c855481b638a7789ad5600b5b283680/specs/kbproto/appD.xml#L732.
This error most likely was accidentally introduced as a copy-paste error in edeb033f.
Fixes #1121 (closed).