It all depends on what you mean by a factory part.
There were Regular Production Options (RPOs) that were ordered by the dealer and installed by GM at their factory.
There were GM accessories that were sold by GM but not always produced by GM (is this a factory part?). These were not installed in the factory and not made in a GM factory. These might also be called GM after-market accessories.
There were, of course, also non-GM after-market accessories.
In 1953, there was a
GM accessory turn signal kit sold by GM dealers but not installed in the GM factory (they might or might not have been made in a GM factory).
One might make a distinction between a GM factory installed item and a GM accessory item (a distinction made by GM, perhaps not by the VCCA?).
