No dating by serial number as it is scratched off.
DATING FENDER AMPLIFIERS (INCLUDING SILVERFACE AND BLACKFACE AMPS) USING TRANSFORMER CODES
In general, Fender amps that don't have rubber-stamped tube sticker date codes have EIA numbers on their transformers that might enable determination of the production date. These numbers always begin with "EIA 606," and are followed by three or four digits in various combinations.
If three digits are present, the first digit refers to the year (i.e., a "7" would mean 1967). If four digits are present, the first two digits refer to the year (i.e., a "66" would mean 1966). The last two digits refer to the week of the year (i.e., a "26" would mean the 26th week or, roughly, June). For example, EIA 606-4-21 would denote the 21st week of 1964. It is also advisable to add about 6 months to the date to allow for time these parts may have spent in inventory storage.REFERENCE MATERIALS FOR DATING FENDER AMPLIFIERS
Hard to picture scratching out a stamped chassis number. Was it covered w/ a serial number sticker? They did that sometimes...usually Showman/Twin though. I ask because a query here not long ago asked about a scratched factory 'sticker' number w/ another number underneath.
@Jason medine606-828 what does this mean on my amp transformer