The string retainer bar can be fitted on most guitars behind the nut or locking nut.
The purpose of the string bar is to force the strings down giving them better contact with the nut or lock nut.
This in turn will improve the general tone of the string.
The strings pass over the nut, under the retainer bar and then on to the machine heads.
The retainer bar is held in position by two screws. The screwsfit between the top pair and the bittom pair of strings.