Comment 46 for bug 261573

From what I have found looking around that weird symbol ( ⌅ ) is supposed to be "Enter" (as opposed to Return) and that seems to follow with how it functions as you stated already (and why that mark is also on the Return key) and also matches what is found on other language keyboards. I believe that most people remap the right CMD/Apple Key to AltGr with xmodmap.

I have also found in some places that on Mac portables, that the AltGr function was obtained by doing Fn+Alt, so you might try that as well. (Is this how it functions under OSX?).

I thought that the omission of the AltGr on the right side is likely due to the condensed keyboard, but I found this image on wikipedia.
http://upload.wikimedia.org/wikipedia/commons/b/b3/Apple_wired_thin_keyboard-2007-08-11.jpg
Notice on the "full" German Apple Keyboard that there is no ⌅ markings, and still no AltGr key that I can tell.

Hope this helps.