Mythbusting 
The serial number of old Washburns
does not indicate year of manufacture.
For that matter, if the first two digits of a four-digit serial number indicate year, that means only 100 instruments could have been built that year -- for instance, #1700 through #1799. This might happen with a small luthier, but Lyon & Healy (makers of Washburn) were at one time turning out more than 100,000 instruments a year, so "small" definitely doesn't apply!