Both rare cars could theoretically be yours for around $50,000 before auction fees. Citroen jumped aboard too, only to eventually decide its rotary GS was something it would rather forget. 1973 Citroën GS BirotorThe regular GS was already wilfully unconventional, with slippery styling and Citroen’s famous hydropneumatic suspension. Rather than keep supplying parts for a tiny population of troublesome cars, Citroen attempted to buy Birotors back and scrap them. 1973 Citroën GS BirotorBroad ArrowBuy The M35 PrototypeBut for Citroen geeks, this Monterey find gets even more tempting.