1.6.4 and 1.6.3 do not, exist, and cannot exist in the future, because we are past the 1.6 phase. That's why these are 1.7 snapshots.
Nope, 1.6.3 is coming out to fix a couple bugs (notably the one about structures not being saved in the world file.) I don't exactly remember which update it was, but I think they released a 1.3.2 after they were already releasing 1.4 snapshots, so it has happened in the past.
Rollback Post to RevisionRollBack
One of these days I'm gonna think of something clever to put in this signature. One of these days I tell you....
Well, this snapshot is rather disappointing, it's purely cosmetic. If they do keep these in the game for 1.7, I at least hope that they create an API for easily adding in custom shaders, maybe they could build it into resource packs in the future. *hint* Mojang *hint*
They already ARE in the resource packs folder. So that means we can already make our own! Seen a few already mess with them.