Abstract: Each arm of a first pair of arms is pivotally connected at spaced-apart pivot locations to a first clamp bracket and to an adjustment bracket, respectively, and each arm of a second pair of arms is pivotally connected at spaced-apart locations to a second clamp bracket and to the adjustment bracket, respectively. A first control is engageable with an arm of the first pair for selectively enabling and preventing pivoting movement thereof, and a second control is engageable with an arm of the second pair for selectively enabling and preventing pivoting movement thereof. When the first and second controls both enable the respective pivoting movements, the second clamp bracket can be moved to a desired position, and, when the first and second controls both prevent the respective pivoting movement, the second clamp bracket retains its position.