Well, André, by shamelessly submitting the answer you've already provided yourself, I can confirm that "hammer spur" is the correct translation for "duimspanner".

As your context suggests, it is the part of the hammer (or cock) that sticks out of the back of the weapon and can thus be operated with the thumb to cock the gun.

Various refs on the web, e.g.:

"The hammer spur is the curved metal piece that extends from the back of the gun and is used to cock a revolver for single-action shots."
http://www.ridersforjustice.com/Articles/FirstGun.pdf

"There are three styles of double action revolver: those with an exposed hammer spur (the device used to cock and de-cock the gun manually), shrouded hammer and hammerless. The hammer may be used to manually cock the gun, allowing for a single-action shot."
http://www.f-r-i.com/women/revs.htm