Having worked in a factory (not firearm) a long time ago I know that all sub assemblies were made separately, sometimes in different plants. They were then racked and stored for use on the assembly line. Some, already serialized, were put together and passed down to quality control (Where I worked). Anything flunking went back to be either adjusted or stripped down for reassembly. I don't know if it is the same for firearms, but I'm told by a friend who has toured the Fender and Gibson plants that it is the same for guitars. Since, unlike some European manufacturers, most makers don't serialize every part, just the frame, it doesn't have to get done on a finished weapon. So when is the gun manufactured. When the frame was made? When it was fully assembled? When it finally passed quality control? Serious time lapses could occur between any of those events.
I also know that you could once upon a time buy several different Colt guns with the same serial number. My first 6" blue Python had the same serial number as a 4" nickel Trouper that was reported stolen in Fa, if memory serves.
One other thing a gunsmith was telling me, before being forced to serialize the Topper shotguns they were produced with over two dozen different firing pins. No way to know which one the gun with the broken firing pin had if you didn't have all the pieces.
Kodak used a date code on their plastic cameras with a four letter code, first two month second two year.