I'm not certain, but I think showkey
is for texmode (non-GUI).
xmodmap -pke | grep Control
keycode 37 = Control_L NoSymbol Control_L
keycode 105 = Control_R NoSymbol Control_R
shows that keycode 105
maps to Control_R (they key next to left arrow).
By the way, xev
will show "X events" (like keypresses). Xmodmap will also accept keycode definitions on the commandline, without having to modify /etc/X11/Xmodmap, restart X, etc.
xmodmap keycode 105 = Left
then, if it works OK, you can add it to /etc/X11/Xmodmap.
P.S. It's been a while since I've had to mess with xmodmap, so I don't know how much (more) help I can give.