Harmuth describes in "Die Armbrust" different types of sights.
The ones seen in the italian video above are a help to control elevation though he mentiones to have a mark on the belly of the prod instead a special attachment like with those sporting weapons.
I would assume sights were common with high quality hunting crossbows, too.
Sensfelder writes in the book fo the Netherlands Royal Army Museum Collection about some of the crossbows with hints to sights but mostly later ones (18-19th century).
The thumbthing is quite easy, the point is to make shots as constant as they can be, so with dents (or similar hints) in different positions in your tiller where you can put your thumb in and afterwards laying your cheek on, you can regulate elevation.
With this bolt length has to be equal, too.