19 January 1917 Count von Bernstorff transmits the Zimmermann telegram to von Eckhardt, the German envoy in Mexico City in the older and simpler German 13040 code. The newer code 0075 was being sent to German embassies one at a time, and von Eckhardt in Mexico City did not yet have the newer diplomatic code.
This was a break for the British because they had completely broken the 13040 code earlier in the war (the code had been in use since 1908). And since the 13040 code was older and known to the Americans, if they could intercept Bernstorff's telegram to Mexico City, the British could safely show the telegram from Bernstorff to von Eckhardt to the Americans without giving away the fact that they'd also nearly broken code 0075.
Later, in early February, this is exactly what the British did.
