DY: I am not sure what you are referring to about “shlissel challah” (BTW schissel=pail; schlissel=key). The Sefer Taajmei Haminhagim states that it is a “remez” back to the time when Klal Ysroel entered Eretz Yisroel. Since the mohn stopped on Tes Zayin Nissan, Klal Yisroel had to rely on there food supply “bderech hatevah” and it is HKBH who holds the “keys to parsanah”.
http://www.hebrewbooks.org/pdfpager.aspx?req=38691&st=&pgnum=151 (hebrew)
http://www.hebrewbooks.org/pdfpager.aspx?req=36722&st=&pgnum=62 (yiddish)
http://www.hebrewbooks.org/pdfpager.aspx?req=36722&st=&pgnum=63 (yiddish)