Available as an option on the Citroen SM after September of 1971, Michelin's advanced Résine Renforcée (RR) wheel is one of the most unique automotive wheels ever placed on the market. Weighting in at 9.5 pounds, these fiberglass-reinforced resin wheels are 11.5 pounds lighter than the standard steel wheel. Metallic inserts on the studs prevents damage when tightening the nuts. Cars with this option also came equipped with special wheel hubs and nuts. The use of this wheels was only possible because the Citroen SM had inboard brakes which kept the heat well away from the wheels. Although at first glance these wheels may appear fragile, their durability was well established by Citroen rallying efforts. Until the recent introduction of Carbon Fiber aftermarket wheels these were probably the most sophisticated non-metallic wheels available.
Note: This set of wheels went for $7,798 USD(5,736 euros) at a recent auction.
All these photos appear to be a set of "version 2" wheels denoted by their rounded outer rim.