Hello Joystick,
you're absolutely right by pointing out that in modern Yiddish the word שמאָנצעס‎ (YIVO: shmontses)  is a pluraletantum. But in the time of borrowing this word from Western Yiddish, there used to be a singular form as well. - best regards, Caligari ƆɐƀïиϠ 14:52, 24. Jun 2009 (MESZ)

Hi Joystick,
the source is already given in the aforesaid article. According to {{Literaturliste|Kluge|sup=24}} the singular form was borrowed from Western Yiddish. - Kind regards, Caligari ƆɐƀïиϠ 23:42, 24. Jun 2009 (MESZ)