The 'M' stands for Modified according to the factory. In the new model, Hasselblad decided to make an improved automatic back holder system (A-series back holders) and an interchangeable focal screen. The Hasselblad 500 C/M gained a lot of fame among photographers because it was a sturdy, robust camera with amazing output and great lenses. Since this time NASA also started using Hasselblad for their photography in space. The Hasselblad 500 C/M needs a back holder to load film.