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The locking bar fits between the two halves of the masada folding stock. Simply place the bar in the stock, set the stock to the desired length, and tighten down the screw. The stock will maintain the set length even with a sling attached to the rear QD mount. The stock will still be able to fold to the side but you will not be able to shorten or lengthen the stock without first loosening the screw.
The "R" embossed into the bar notes which side is to be facing the rear of the stock.