If I had to guess, I'd say the "stomachs" you're buying in Germany are what we would call "gizzards." The gizzard is a muscular organ that some birds use to grind seeds. A chicken gizzard is about one inch in diameter and looks like a solid blob of pink muscle. A turkey gizzard is about twice as big.
I use chicken gizzards as muscle meat fairly often--they are cheap here (I can buy enormous "family packs" of them at stores that have lots of African American customers.) And nothing to chop or slice.