If you lost the knob it may have been one without the brass insert and simply slipped off. Talk to your dealer. I believe warranty will send you one.
We have received a few reports of failures to H-Rail accessory tension knobs.

There is some variance in the threaded diameter of knobs which may cause the knob not to have enough gripping strength. Some knobs have slipped under load while many others perform as designed. They can easily be tested by putting them on a rail, and tightening the knob/latch down. The likely result of the loose threads is that the clamp simply slips a little, so a catastrophic failure with the rod rack, drink holder, or universal mount is unlikely.
The knob’s plastic threaded hole is being changed to a threaded brass insert which is molded into the plastic to provide consistent strength. We’re looking at early December for knobs to be in your hands.
If you would like new knobs, please contact your dealer if you have purchased a 2015 Pro Angler or any H-Rail accessories so we can send out replacement knobs when they become available.
84620111 KNOB, REPLACEMENT H-RAIL CLAMP