https://www.aulro.com/afvb/images/im...014/11/135.jpg
Not seemingly shown is the bracket in the back corner of the of the door-jam.
Also on the late SIII 109 and Stage 1 the inertia reel was mounted below the gal capping of the seat transom where there was a doubling aluminium panel welded to the back side. You could use the usual steel reinforcing plate in the same place.
https://www.aulro.com/afvb/images/im...2016/04/91.jpg

