Finding the right modifier ID in a japanese keyboard layout

[xorg at henrik.synth.no]

Hi.

I've got a small laptop with japanese keyboard layout, like this:

http://www.notebookreview.com/picture.asp?f=27325

To get AltGr functioning, I've mapped the key just right of the space 
bar with xmodmap:

xmodmap -e "keycode 129 = ISO_Level3_Shift"

This works, but as I understand, xmodmap is out of style.  As I 
understand, I should do something in XkbOptions in xorg.conf, or make my 
own symbols definition in /etc/X11/xkb/symbols/

The problem is, what symbol is the key just right of the space bar?  The 
alfanumerical keys are easy to name (AE01 etc), but this one (and the 
other japanese modifier keys) is considered special.

Does anyone know?

Best regards,
Henrik