Thank you for posting the pictures!
I think I finally have an answer to the question I have asked in the past, but was never answered.
From looking at the picture -


it appears there are 2 notches in the hammer for the trigger sear to engage, providing a half-cock.
I have never seen a underhammer action in the half-cock position and wondered if they had one.

I like that sight, too! Looks very simple, but functional!