The idea of them being secret codes is bit goofy, especially when some of the things have both the code and the name of the maker on them. This is especially true with electronic devices.
The allies had very good estimates on German war production. This was especially true with your higher value items, like planes and tanks. It even lead to a theory:
https://en.m.wikipedia.org/wiki/German_tank_problem
The one thing that the allies did mess up on calculating was the number of production machines like lathes and mills. The big mistake was based on an assumption that the Germans would rationally run their war economy.
Let’s say one machine can make 100 parts in a 24 hour day. If you find 500 objects made in one day, you know that five machine worked to generate those parts. Get enough data points and you can figure out a nation’s industrial capacity.
HOWEVER: the Germans were only doing 10-12 hour days because of the unions and other labor issues. The rest of the time the machines were sitting dormant, doing nothing; not making war supplies. Allied nations were doing 24 runs and just assumed that the Germans would do the same thing. When allied teams went to Germany postwar they were blown away, as early all machine numbers had to be doubled. They couldn’t believe what they found. Just astonishing.