Oreste, we've got several side car guys on the forum, they're not on everyday but will be along eventually.
2021 CVO TriGlide - Sunset Orange & Black
Life's journey is not to arrive at the grave in a well-preserved body, but to skid in sideways, totally worn out, shouting...Holy Sh!t What a ride!
I don't know alot about side cars but that looks like one made for racing.
Stallion #406 // 2013 Tri-Glide
I think Watsonin had something similar years ago. But that looks home grown to me. It would be for trials riding. Not to be confused with trail riding. The pop rivets and their installation spacing don't look factory made. Kinda cool if ya have an old English thumper. Quite nicely done. A one off built specific to the bike he had. As you can see no adjustments, and no clevises.