The following paragraph is from Dr. Asher Eder’s book The Star of David, which was published in 1987 in English in Jerusalem by Rubin Mass Ltd. The publication here is courtesy of Oren Mass
This version includes corrections and new materials that do not appear on the printed version
The famous Stadt-Kasino of Basel where the Congress took place had the six-pointed star above its entrance, and some meters away from it a white flag with two stripes, probably in dark blue.
The Delegirten-Karte (delegate's card) for the Second Zionist Congress in 1898, also in Basel, was decorated with a Magen David showing a lion in its middle field and seven five-pointed stars resembling the manner shown above (fig.19), embellished by two decorative stripes.
Meanwhile, the symbol had made its way to the United States. There, the American Jewish Publication Society adopted it as its emblem in 1873, while it was flown as a flag for the first time in 1904 at the St. Louis World Fair, in the design we know today.
The inventor of this flag may have been motivated, if not inspired, by the design of the Delegirten-Karte for the Second Zionist Congress.
Dr. D. Wolfsohn, a close friend and successor to Dr. Theodore Herzl, then suggested that the flag of Zion consist of the Star of David with the two parallel lines of the talith (the Jewish prayer shawl) above and below it. This suggestion might have been modeled on the aforementioned delegation card, and perhaps on the flag used in St. Louis (Missouri), although we have no proof of this.
Next, the Star of David graced the flag of the first Jewish army unit in modern history, the Mule Corps of World War I.
However, some decades passed until the 18th Zionist Congress approved this design as the flag of the Zionist Organisation and the Jewish people. The Congress, which took place in Prague in August 1933, put this sign out as a banner, thereby defying the slander which had become the official policy of Nazi Germany.