Six-digit serial numbers with no preceding letters, with the first number being the year. CY appears on most models produced in China, and is by far the most common form.
Squier II's could not possibly have been assembled before 1987/88, as these represent the transition to Indian and then Korean production, and all the ramifications of such (the serial numbers can be off by years for these particular axes, and neck dates may also be misleading). This form is associated with Made in China Affinitys (with the standard body thickness of 45mm/1.75). These forms are apparently associated with the change to Crafted in China, so post-1996 models only would have this form of serial number. YN is associated with 'made in china' rather than 'crafted in china' and 1996 was the year they swapped to 'crafted in china' and mostly CY prefixes.
Given the modular nature of Fender's production techniques, an individual neck may have been produced in a given year, placed in the manufacturing 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.
So, obviously a neck date, while being helpful in providing a date range of production, it cannot be a definitive reference.
Some Chinese made Gretsch guitars also have a CY serial number.