This too is only a guess, but perhaps Spiegel is being used here to refer to a smooth surface in the sense of a mirror finish or mirror polish finish. A smooth surface of some kind certainly seems to be the idea.

I've found another patent relating to a rasp, which refers to rounding or smoothing one end of the rasp to accommodate the hand.

[0018] The palm rest can be made integrally with the rasp surface by folding back the metal to form a nicely rounded edge without protrusions. The end can be otherwise rounded, for example, a separate palm rest can be welded thereto or the edges can be smoothed by filling and/or grinding. However, in a preferred embodiment, the palm rest is formed integrally with the plastic handle cover, thus minimizing the weight of the rasp and providing a smooth and comfortable surface for hand contact.
http://www.freshpatents.com/Convex-rasp-dt20080103ptan200800...