Truss Rod Covers - Four unchanged OEM covers

Notice anything odd about the Harmony truss rod covers below?

The holes are all in different locations. Some variations are dramatic. So, a question that arises is, were these drilled in place by hand? Variable hole locations are more common than not. Problem is, the plastic they used is exceptionally fragile. If you just stick screws into holes that kinda fit but not perfect, you will almost certainly break out the screw holes. So, it is safe to say that in most instances, the truss rod cover is specific to a particular guitar. They are not generally compatible from guitar to guitar. At least, great care is required to screw down on another guitar. You need to check alighnment of the cover holes with the holes in the gear head.

My theory is that this plastic was a late fifties early sixties product. Unlike most modern hard plastics, this plastic degrades over time. The same is true of toy train plastics from the same era. So these little devils are very fragile.