The Summicron was the first Leitz lens to use LANTHANUM crown glass. Normal crown glass has been in regular use in optics since the 1730s; without it, there would have been no achromats until the 1950s.
"Summicron" is a progression of the Summar/Summitar/Summarit naming convention for double-Gauss normals. If you can get -cron out of "lanthanum", I salute you. :P
f/2 is the middle speed of Leica's lineup, between 2.8/2.5 and 1.4/1.0/0.95. All 'Crons are f/2, and all 'Crons are also mid-speed.
Most M-mount lenses don't have -M in their names. That's a recent convention. A Summicron 50 f/2 and a SUMMICRON-M 50 f/2 are not the same lens, either optically or mechanically.