This is a tough one, and short of various inspection stickers, and reliable transformer codes and pot codes, there is currently no definitively reliable way to determine an accurate date based on serial numbers alone, with the possible exceptions of the DMI-era products and beyond. The serial numbers during this era appeared as 74001234 or 78115422, as just two examples. (Here’s an example of a speaker cab’s ID plate.) The two-digit prefix in these serial numbers seems to indicate the year made, and in the latter example, which was seen on a DMI-Sound City F115 bass cabinet, it appears the model number might have been included in the serial number as well. For the earlier Dallas Arbiter-era Sound City products, however, the best you can legitimately determine is a range of years during which an amp or cabinet was made based on the eras in which these products appeared. (See also “Q3” below.) These eras and their approximate and relative date ranges are as follows: – Arbiter Electronics