Putting a date on Strat pickups

[shwingdom]

Hi all... I am trying to get a rough date on a set of Strat pickups that I have. They are single coil with staggered poles and have a "Patent Applied For" label on them. I've had them for 20 years and I believe the guitar was a late 70's model.
Hope someone can shed some light. Thank you.....Jim