Some of the Otto Keramik labels are plain paper, not foil. They look like they may have been done on a typewriter, appearing rather like fortune cookie inserts. Sadly, these don't tend to stay on very well.
Underneath the felt, Otto Keramik bottoms have a white clay and flat base, just in case you come across one without the felt. I had a marvelous vase several years ago and spent a great deal of time and effort removing that felt to check for a mark since this was before I knew Otto Keramik existed, much less how to identify it. Talk about a vase I'd like to have back. It was 10 or 12" tall. You can see it and an uncovered bottom at:
http://www.ginforsodditiques.com/assorted.gallery.htmlAlso, check out the Otto Keramik purple glaze below that one. Meanwhile, I have to go in and correct some misattributions since I haven't updated those pages in a while. Otherwise, Kevin will start making fun of me.
Forrest