The 2.05 Extras disk includes the keymaps usa0 and usa3, which correspond to the American A1000 and A600 keyboards, respectively. usa0 can also be found in 2.04, but I don't think usa3 is on any other disk. I don't know what these offer over the regular American keymap, but perhaps this info is useful to someone.
I've never found a usa1 keymap file--I assume this is the default American keymap and is part of the ROM.