This is quite an interesting document/label I learned about in the Apple Service Technical Procedures Volume Four Binder. Apparently back in the day Service centers had replacement units on hand with no serial numbers. If a customer brought in a defective device or one needing to be entirely replaced these service centers would simply take one of these unserial numbered units and using this label they would write your serial number and apply it to your device. This is why on some vintage apple monitors or devices you see a handwritten serial number, this being one such example of many. Preserved and saved for your use, enjoy.