We don't know, but conveniently enough, 1985 was a rather defining year in Fender's history, so we're going to go ahead and say so. Most notably, production dates have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses, although there were periods when this was not consistently done (1973 to 1981, for example) or simply omitted.
Dating a Fender Guitar by it’s Serial Number can be a little tricky.
In most cases you can only get a ball park range for your production date.
Most notably, PRODUCTION DATES have been penciled or stamped on the butt end of the heel of the neck of most guitars and basses, although there were periods when this was not consistently done (1973 to 1981, for example) or simply omitted.
instrument production history, PRODUCTION DATES have been applied to various components.
The neck date simply refers to the date that the neck was produced.
Given the modular nature of Fender's production techniques, a neck may have been produced in one year, placed in a warehouse and remained in stock for a period of time, and then subsequently paired with a body to create a complete guitar in the following year.
Given the modular nature of Fender production techniques, an individual neck may have been produced in a given year, then stored for a period of time before being paired with a body to create a complete guitar, perhaps, for example, in the following year.
Therefore, while helpful in determining a of production dates, a neck date is obviously not a precisely definitive reference.
A guitar neck could have been produced in 1956, then slapped onto a body in 1957 and if late enough in the year pickups may have been put in early 1958.
The best option for getting a close production date is to remove the neck from the body.
So, obviously a neck date, while being helpful in providing a date range of production, cannot be a definitive reference.