You should be able to take a pair of tweezers or peg wood and slide this hook outside. There should also be another hook just like it on the opposite side of the movement. Just be careful not to damage the mainplate in the process.
You can also find the technical document for the ETA 2671 here:
https://secure.eta.ch/CSP/DesktopDefault.aspx?tabindex=1&tabid=28
Just look under mescaline for product range and 2671 for the caliber.