The clamp eraser was not only a feature of many Eberhard Faber products, its design was extolled on some of their packaging too. As you push the end of this box, a cardboard insert shaped like the eraser extends outward from the top:
I was surprised (and interested) to discover that there is a separate patent just for this box. From the description:
“One object of the invention is to provide a box of the general character described having improved and novel means for illustrating the construction, operation, or characteristics of the articles packed in the box.”
Could this be the world’s first (and only) recursive pencil box?
It dates back to 1930, which is nine years after the eraser was patented. As of this writing I have only seen Van Dyke boxes with this design, but you’d think that there must have been an early Blackwing box like this too.