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.
Pretty sure that the "icron" suffix was used after the canonical summ prefix to denote how (now famously) sharp the lens was. Sum-MICRON, as in resolving detail down to the micron level.