German tanks during WWII were far superior to the Allies’ tanks. Since they couldn’t beat their tanks, the Allies wanted to at least know how many the Germans had. Their original estimate was that German industrial complexes were producing 1,400 tanks per month! That was a huge number and caused the Allies to doubt their math.

They realized from careful examination of German tanks they had captured that each had a unique and specific serial number. Within the serial number was their order of tank production. The Allies could figure the number of tanks from the serial numbers. They were able to establish that the Germans were only producing 255 tanks per month.Nothing compared to the original idea of 1,400 tanks per month.  

(Source)