A reason Fender used grey flatwork?

[Spence]

They were die cut in house with dies made by Race & Olmstead.

[Chadheckler1]

In fact, it might make it less prone to error, for example having tops cut from the thick stuff by mistake.

That explanation makes the most sense to me now that Spense confirms that they were cut in house...kind of a visual pok-e-yoke. So the guy punching the tops would use the black stuff and knew that it was the right thickness without measuring it and the guy punching the bottoms would use the grey? You would have a hard time screwing that up even if you were in a hurry...