Kustermann Seal, Image by StuE, Found by Quozzy.
Found in Germany, 15mm, 4.1g.
Appears to have the same markings on both sides - inside a circular border KUSTERMANN curved downwards at the top and MUNCHEN curved upwards at the bottom. There may be further markings in the centre but they are too faint to distinguish on this seal.
Kustermann of Munich have been well known traders mainly in hardwear since 1798. See Kustermann's history.