Two different designs, early 1X9's had a cast iron support with a 1/4" bolt supplying clamping force on the rob the speed/direction lever turns, lighten very slightly. You can reach the bolt by removing the center frame cover between the seat and steering console.
Later 1X9's used a threaded nut that clamped down on a stationary support plate, tighten the nut a bit, access same as the above earlier style. My 129 was the earlier style, I haven't had any experience with the later 1X9's.
You can access parts books and owners manuals at IHCUBCADET.COM. From title page, hit the bold text "IH CUB CADET FORUM" and manuals will be at the bottom of index marked "Manuals".