They can be cast or stamped into a brass tag that is soldered or screwed onto the case. Some machines have two tags, one for the serial number, and another for the model number.
Most machines have one tag with both the model and serial number stamped into it.
These tables will help you determine when your National Cash Register was built or rebuilt. Look under rebuilt equipment for dates on second hand registers. If there are any letters after the serial number on a second hand register. You will be able to look up the original serial number, and then the date of manufacture. Look under symbols for rebuilt equipment.