I found this
From Avanti Magazine Issue 187 Summer/Fall 2019 article by Curtis Fischbach AOAI Michigan member.
The 1987, 1988, 1989, and 1991 Avanti convertibles have many common parts.
The hydraulic power unit is from the 1987-1989 Chevrolet Cavalier (Pontiac Sunbird). It was produced by the Power Systems Division of Dura Corporation. Dura Corporation became the Dura Convertible Systems Division of Wickes Companies, Inc. Wickes, later Collins and Aikman, was liquidated in bankruptcy in 2007. Rebuilt replacement units are available in the aftermarket.
The header, or Number One bow, is a sand casting, produced by Total industries, Inc., of South Bend. Those headers were made in batches. After the production completed, Total Industries, Inc. kept the mold for many years, but no additional pieces were ever made.
The latch assemblies mounted in the header were from ASC, Inc., of Southgate, Michigan. The handles were shortened since the Avanti was narrower than the cars ASC, Inc. had made them for. Most of these latch parts could be found in the Renault Alliance convertibles.
The side rail assemblies, those convertible geometry devices that make the convertible fold, are 1987-1989 Chevrolet Cavalier units, shortened at the front to fit the Avanti and its header.
The remaining bows were again the 1987-1989 Chevrolet Cavalier bows, cut in the middle, shortened to fit the Avanti, and welded back together.
The side quarter windows, between the front door glass and the convertible, were 1987-1989 Chevrolet Cavalier quarter glass along with the seal that meets the front door glass.
The seals across the header and along the window line were aftermarket seal material, fashioned to match the seal designs used in the 1987-1989 Chrysler Le Baron and 1987-1989 Chevrolet Cavalier.
The seal carriers for the side windows were from the Chevrolet Cavaliers as well.