Mmmmm...

This is the bracket that screws onto the bottom of the seat.
https://www.bacchettabikes.com/produ...uromesh-seats/

This is the seat clamp that slides along the tube.
https://www.bacchettabikes.com/produ...amp-bacchetta/

The notches in the brackets go onto those sticking out bolts and then you tighten the bolts.

If you have a Bacchetta seat with the brackets on already then you can make yourself a square sliding clamp that you can screw such bolts into.

Otherwise you can see how the brackets bolt to the seat base and you can create whatever attachment to bolt into those holes on the seat.

All different seats have different sizes and methods of attaching, so you need the seat really to measure and work out the fine details.