I just looked at my Piaggio top case mount and yes, it's different than the 1 we are trying to match, but I've seen a top case just like my Piaggios either on a Norge or Aprilia bike recently, and my mount doesn't look much like yours. It's narrower than the 1 we're trying to match, but it has the tower/locking feature like the 1 we're trying to match.

Since Piaggio owns Aprilia and Moto Guzzi too................