Europe and the Pacific were the two primary arenas in World War II, but the battle also was one of technological achievement. Airplanes, submarines, and of course the development of the atomic bomb were core markers to the arms race within the War. Similar, tank warfare was as much a function of innovation as it was anything else. Allied forces readily admitted that the enemy German tanks, such as the Tiger and Panther, were superior to their own. The big question for Allied forces, then, was not how to counter the Tiger and its progeny, but how many tanks Germany was able to produce.

To solve this problem, the Allies first turned to conventional intelligence gathering: spying, intercepting and decoding transmissions, interrogating captured enemies, etc. Via this method, the Allies deduced that from June 1940 to September 1942, the German military industrial complex was churning out roughly 1,400 tanks each month — an enormous amount. By comparison, the Axis forces used “only” 1,200 tanks during the Battle of Stalingrad, an eight month battle with a total casualty count nearing 2 million people.

Perhaps skeptical of the above result, the Allies looked for other methods of estimation. And then they found a critical clue: serial numbers. As reported by the Guardian, Allied intelligence noticed that each captured German tank (and wreckage thereof) contained a serial number unique to that tank. With a bit of careful observation, the Allies were able to determine that the serial numbers had a pattern denoting the order of tank production.

Using this data, the Allies were able to create a mathematical model to determine the rate of German tank production, and estimated that, during the same summer 1940 to fall 1942 time period, the Germans produced 255 tanks per month — a fraction of the 1,400 estimate produced by conventional intelligence. (Want to see the math? Click here.) And it turns out, this method worked best: after the War, internal German data put the number at 256 tanks per month.

Bonus fact : Both math and language evolved during the Second World War. For example the term “snafu” most likely comes from the time period. It’s widely believed to be an acronym first used by U.S. soldiers during the War, standing for “situation normal: all f*cked up.”

From the Archives: When the Nazi’s Invaded America: No tanks. And no success, either.

Related: Toy models of German tanks, to scale.