Polycarbonate signals are popular in a number of regions. They aren't as rugged as aluminum but they are less expensive and weigh much less than similarly designed aluminum signals. Therefore polycarbonate occasionally has some real advantages.
This particular signal is an Eagle Durasig 5-section protected left turn signal. It would typically be post mounted as it would be too long to hang from a span wire or mast arm. (A dog house signal would ordinarily be used for an overhead installation.)
There' not much to restore with poly signals. This one just mainly needed cleaning, be wall mounted and wired up to the master controller.