Maxon made pickups of the late 70's and early 80's are widely acknowledged as some of the best used in vintage Japanese guitars. Late 1970's Navigators were sometimes equipped Maxon Humbuckers stamped with an 'S'. While fairly rare, they are considered to be an outstanding humbucker by those that have experienced them, which I would agree with.
I recently acquired a 1978 Navigator Gold Top equipped with its original P-90's and was surprised to find that these pickups were made by Maxon and had the same 'S' stamp as the humbuckers.
I could find no references to these pickups, so it appears to be the first set found. I happened to have a 1980 Greco EG 800GS which came with 'Hot Lick' P-90's which were made by Maxon for Greco in 1980 so I compared the resistance on both. Neck pickups measure 8.00 on the Navigator and 8.06 on the Greco. Bridge measure 8.31 on the Navigator and 8.19 on the Greco.