For Stratocasters it is 22 and for Precision Basses it is 5.
The other three digits (320) are a mystery and perhaps are some kind of batch or lot number. Here’s our P-Bass again (s/n 277983) with the code 529129B.
One plausible explanation is the use of leftover necks – Fender is known to have done this often.
If you have a guitar or bass with this first style green code and you would like to submit it to the database I’m compiling, please send the model, serial number, code, and if you have them, the pot dates and pickup dates.
An example of this is 02033923 found on a Jazz Bass.
From 1972 through about March 1973, this new system was used concurrently with the old Month-Year date stamp.
The new code consisted of 6, 7 or 8 digits and was usually stamped in green ink.