GM and other companies sold accessory directional signals with different styles of fender mounted lamps that look like yours. Some had a lens on one side and some had lenses on two sides.

When I went junk-yarding in VA in the 60s and 70s, a majority of the trucks in the junk yards had similar directional lamps mounted on the fenders.

6v and 12v replacement bulbs should be readily available at a good parts store. As Tiny suggests, take out a bulb and take it to a NAPA or other parts store.