Default xmodmap modifier map under Xubuntu 10.10; notice odd sets of keys in mod1 and mod4

Attempt to apply my own modmap; note BadValue error and persistence of some of the original strange modifiers

0 bournemouth:[ ~ ] $ cat .Xmodmap-manual 
! I started with code from
! <http://tylerkieft.com/archives/2006/10/05/redefine-the-x11-meta-key-in-mac-os-x/>,
! but by now that's unrecognizable here.
clear Mod1
clear Mod2
clear Mod3
clear Mod4
clear Mod5

!keycode 63 = Mode_switch
! Super is used for the Windows key in Ubuntu
! 63 is left Cmd on MacBook
!keycode 63 = Alt_L
! 71 is right Cmd  on MacBook
!keycode 71 = Alt_R
! 66 is (left) Option on MacBook
!keycode 66 = Meta_L
! 133 is left Windows on PC
keycode 133 = Meta_L
!keysym Alt_L = Meta_L

add Mod1 = Meta_L Meta_R
add Mod4 = Alt_L Alt_R
! I'm not sure why this was set to Mode_switch. I think that's used for Num
! Lock, but I'm not sure.
add Mod2 = Mode_switch

0 bournemouth:[ ~ ] $ xmodmap .Xmodmap-manual 
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  14
  Current serial number in output stream:  14
0 bournemouth:[ ~ ] $ xmodmap -pm
xmodmap:  up to 3 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x25)
control     Control_L (0x42),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3      
mod4        Meta_L (0x85),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)

xmodmap-pm

0 bournemouth:[ ~ ] $ xmodmap -pm
xmodmap:  up to 3 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x25)
control     Control_L (0x42),  Control_R (0x69)
mod1        Alt_L (0x40),  Alt_R (0x6c),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3      
mod4        Super_L (0x85),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)