It was a "flanger sign". They indicated when plows (flangers) should be lifted because a crossing had low clearance between the rails.
Here's what it looked like:
http://prr.railfan.net/standards/standa ... an=78460-A
Since there were no signal circuits on the branch, the choices are pretty limited. The plate shows the roadway over the crossing filled, with timbers perhaps. That would require such a signal if its clearance was high.
By the way, the cast iron signs weighed 15-1/2 pounds each and were mounted on a 1-1/2" diameter iron pipe.