The Leitz 50mm lens went through many different iterations since 1924, starting with the Anastigmat designed by Max Berek … the Summar was the first of the f/2 line in 1932, then Summitar, then finally the Summicron.
The Ernst Leitz Wetzlar Summacron 50mm (5cm) f/2.0 first production M-mount started 1954. The Leica part name for the m-mount version is SOOIC-M (don’t ask me about that). 7 elements in 4 groups … that’s a lotta glass !!! The original version started as a collapsible, later 1956 they also producted a rigid body.
Early 50mm Summicron’s had lens elements, made by Chance Brothers & Co., that had thorium oxide in it … very low radioactivity and it eventually turned the lens little yellow/brown over time … they quickly switched over to their own lanthanum oxide formula.
Marco Cavina has a great website about Leica, though it is in Italian … but I don’t think you will have any problems finding reading matrial about anything Leica.
Now I am not a Leica person, I was caught offguard when people use the suffix to talk about various lenses … and this one being the Summacron 50mm (5cm) f/2.0, it was refered to as a cron. It appears that a Summacron refers to the f/2.0 aperture size ? … obviously I am a Leica newbie … so what do I really know.