There's photos of the manual at -
https://get.google.com/albumarchive/...slEh1HftK6nvSV

Looks like X1-4 are used, however there doesn't appear to be anything that controls the speed. The ribbon cable goes to the remote display, and X terminals are basic inputs. You'll need to go through the menu, and find out what the X terminals are configured to do.
Probably worth testing with a multimeter, and see if any of the X input statuses change above 10'000rpm.