The majority of Fender pickups that are dated are the ones made of light and dark gray vulcanized fiber during the mid 60s on. They used a pencil and then an ink stamp. I’ve seen mid 60s black bottom bobbins stamped with yellow ink and mostly on Fender Mustang and Duo sonic pickups. The best way to date the pickup for an approximate age is to check the neck and body dates along with the Date Source Codes on the potentiometers.