In regards to the thumb trigger, since the weapon was custom made shortly after WWI, a war injury seems the most likely reason for the unconventional construction. As fancy as the weapon is made, the special trigger is not a huge extra expense. It would also go with the rebarrel, the 9.3x74R is ballisticly similar to the 9.3x62, and, from my experience, that one has a decent kick. Someone missing several fingers might have trouble controlling such a gun, so cutting down on recoil by using an older slower round might have been the reason for the rework.
BTW it looks like the horn cap on the forearm is missing.
F 135 - the right choice