Widely used for temporary gate ways or even permanent ones that are not used often enough to warrant spending on a complete new gate and posts. Usually fitted with square sawn rails but 1/2 rounds can be used if small enough or shaped to fit the brackets. 
Locking arms on the bracket can securly lock the rail into place by cutting a small notch in the rail as shown in the photographs.
This items consists of two bracket, one rail slide bracket and one rail slide locking bracket. This allows a single rail to be installed.