JT

Chapuis makes a 12ga side by side Slug Gun.

It is regulated for Brennekes [at 50 meters if I remember correctly].
It has iron sights and the ones I have seen here in the states have the rib precut for scope mounts.

I have handled them and they feel nice.

Also I think it is Eurogun Imports that has the "Sabatti Leopard Gun", a 12 ga SXS, but I do not know if they are regulated for slugs.