A fast normal prime is a key lens in almost any system. For Leica and the M system, this role is best played by the Summicron 50mm f/2 lens. There is a little difference compared to most other systems, though: usually a fast normal prime is also a really affordable lens. Anyone familiar with the brand already knows that there is no such lens in Leica land, at least as long as we're talking about current products. For the price of the Summicron, one could also buy ten 50mm f/1.8 SLR lenses with F or EF mount.
The Summicron 50/2 as it is sold today is already the 4th generation. The first version dates back to 1959, while the current product was introduced in 1979. Since then, the lens has not changed internally, making it one of the oldest products in Leica's portfolio. However, the barrel has been redesigned in 1994, replacing the clip-on hood with an integrated one.