I heard today from Rabbi Moshe Rabovitz of Shaarei Hesed, Jerusalem, that Kosher Butchers in Europe for the last few hundred years always used the Magen David sign outside their establishments to announce that triangular (kosher) meat was inside.
He believes the reason is to be found in the Talmud Baitsa 28:a where it says that it was the practice for Kosher butchers to cut Kosher meat into triangular form so that it could be distinguishable if non-Kosher meat accidentally got mixed up with it.
The Magen David sign is simply two triangles superimposed on each other
I checked it out and found that
Raba the son of Rav Huna used to cut his meat into triangles… he would thus send meat to his wife with a non-Jew and she would be able to identify the meat as coming from her husband by its unusual shape.