This is the molding that runs along the top of the door (bottom of the glass) that the door glass slides against when the window is moved up/down.  These typically get dry rotted (and often break into pieces) over the years.  We only have a limited number of these in stock.  These fit all Maserati TCs - 1989-1991.  This is the LEFT (driver side), however with (very simple) slight modifications you can install these on the passenger side as well, as we have done many this way with 100% success.