Since no official lookup table exists, you must use physical markings and manufacturing history to estimate the age:
You can roughly date your Model 840 based on the following milestones: Can Anyone Date Cooey Shotguns - Year of Mfg. Cooey Model 840 Serial Number Lookup
Q: Are there any online resources for Cooey Model 840 serial number lookup? A: Yes, online forums, communities, and reference books can provide guidance on researching your Cooey Model 840 serial number. Since no official lookup table exists, you must