Despite the fact that they're valuable, and would seem to be easy to forge due to the poor quality of the legitimate issues, there don't tend to be a lot of forgeries out there. Or at least not good ones. Just not enough demand, I guess. Forgers would rather take people's money on fake Tunis feldpost stamps, since the demand for those is always sky high.
All legitimate uses should be on piece. Michel notes that a piece should be considered "complete" only if it has the parcel permission stamp (with or without the 20 Pf), sender and receiver address, and the proper Feldpost markings (usually unit number and Feldpost cancel).
I did a quick search to see what's out there right now, and sure enough, the only two I found used are on piece.

This copy, from which it appears someone removed the additional 20 Pf stamp, went unsold at the last
Christoph Gärtner auction. Starting price was €1,300.

This much smaller piece is being auctioned at the May
Jennes and Klütterman auction. Starting price is €500.