Zwanziger
A name applied to the sil- ver pieces of twenty Kreuzer, or one third of a Gulden, formerly in circulation in Bavaria, Salzburg, Austria, etc. The peas- antry frequently soldered a loop to them and used them as buttons, hence the nick- name " Knopfzwanziger. "
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)