Chiliwailer, who replied to your post in 'bass guitars', is very knowledgeable about vintage Fender. As he says, it's unlikely to be 1966 as the four digit code came later and was used in the early 70s. This was replaced by a 6 digit code from around 1974 so the last digit (which usually indicated the year) being a 6 is a bit of an oddity. Which I guess is why you're asking! Pics showing examples of 1966, 1974 and 1976 pickups attached for ref.