Turn signal sets were available from Chevrolet/GMC dealers and they look very similar to what you have. They are referred to in the Accessory Installation catalogs and also the Salesman Data Books. Here are some pictures of examples of kits one from 1952 for a signal and wiring (lamps were avaialble too, usually Signal-stat or Guide I have some pictures of them somewhere I will look for) and a set of 1951 Fog Lights and fender mounted signals from 1951. So they were dealer installed accessories and therefore very much a factory item.
http://good-times.webshots.com/photo/2098465170102454676JqJmgv
http://good-times.webshots.com/photo/2611169830102454676poCOpg
http://good-times.webshots.com/photo/2251193060102454676gZmuDn
http://good-times.webshots.com/photo/2799557560102454676TdvwPu