The Alternate Graphic key is a PC keyboard key, and seen on some other boxes including on some Sun keyboards. This implies that the keyboard in question is a random German-language PC (or Sun?) keyboard, and not an LK-series keyboard.
Your subsequent response indicates that the central issue is with an unspecified DECwindows (CDE? Motif?), and that the issue arises once the box is booted and running, so I'll ignore the console-level "fun" that can (also) arise with this case. The usual workaround here was for PC keyboards was to set DECwindows on the box to an LK443 keyboard (101 keys) or to an LK444 (102 keys) keyboard, and to see how far you get with that. IIRC, there are some versions of DECwindows also have a PCX-something-or-other keyboard or three offered, too.
With old and weird and unusual keyboards, you'll sometimes need to use the Alt-Space chord to get to the DECwindows Compose sequences.
On the German variation of the typical 102-key PC keyboard, try the AltGr-Q chord to get to the @ character.
You'll almost certainly have issues with keymapping here beyond what you're seeing here, too.
And in the entirely likely case that I've gone off the rails with my assumptions and guessed wrong here, please post up a few more details around the keyboard and the environment.
