There was a seat back and bottom available for the back jump seats. If someone has the european parts book for the early 70's spider maybe they could post a picture of the pages that show the rear parts.
skip, in the USA spiders didn't not come with rear seatbelts, to be so equipped would have been illegal and not authorized by ALFA ROMEO; someone added those in the picture to your friends car. On a 78 the retractors for the front seat belts were mounted in the rear section of the car. Please post a photo of the "bolt heads"; the sway bar mounts have a total of four bolts that protrude from underneath into the rear section.