I hope the above pictures will help you identify the MYSTERY PART. The shower doors are without frames. They are about 28 years old and I do not know the manufacturer. The bottom tract is raised single sided on the bathroom side. The tub side, is without a raised side wall. The length of the piece is 3". The width of the piece is 1 3/16 with an extended upward extension of 4/16 (this fits into the raised bathroom side of the tract). The detached piece (broken off??????) is a semi-circle tract with a division piece separating R & L, I think this is what to guide the two shower doors and prevents them from contacting each other.
Please HELP!
THANKS.
FranĀ