The saddle repeatedly failed to stay in place while riding. Most often, the tilt would change, kicking the front of the saddle upward when weight was placed at the back of the frame. I tried repeatedly tightening the clamp nuts without much effect. The 13mm nuts eventually stripped the cross-bolt.

I replaced them with a ⅜" diameter bolt from the hardware store. The rails on the B135 must be spread slightly for the new bolt to clear, but it works without damage to the rails. I also had to use a taper-reamer to slightly expand the square holes through the clamp itself for the new bolt to clear. However, with the larger bolt & higher torque, the seat now stays put.

Just a FYI...