I know that I have seen the recipe for this drink somewhere. 'Zam Zam' was the name of a ship during World War Two. The bartender on the ship invented the drink which was named after the ship. Here is one report of the event.

Apr. 17 - Neutral Egyptian liner Zam-Zam sunk in S. Atlantic, 140 U.S. aboard - 1 American died of injuries, 2 other seriously wounded passengers remained captive as "Guests of the German Navy" aboard the German merchant raider Atlantis (Hilfskreuzer) due to their injuries for another seven months

The captured Americans were missionaries and tobacco buyers. The tobacco buyers frequented the bar before and after capture, so the bartender invented a special drink to help them endure their captivity. Below is another record of the ship.

Collection (1941-1942) consisting of a photograph album of the S.S. ZamZam, an Egyptian-owned ship, its crew and passengers, including 120 American missionaries (from 21 different denominations), tobacco buyers and other passengers traveling from New York to Alexandria, Egypt, via Capetown, South Africa, who survived sinking by the German raider Tamesis 17 April 1941, including newspaper and magazine clippings, photographs, periodicals, correspondence, and photocopies of an autobiographical account

My problem is that I want to know how to mix a Zam Zam.