PMC.S = "Prince Motor Club. Sports"
The Prince Motor Company, like many of the Japanese car manufacturers in the late Sixties, made their race teams like little supporters clubs. PMC.S was formed as the Prince in-house race team, and fans could join the club and get extra benefits at race meetings and regular newsletters etc.
When Prince were merged with Nissan, PMC.S became a focus point for sporting Skyline fans in particular. The Skyline factions inside the Nissan organisation kept up the traditions of the club and still used the famous white on red PMC.S stickers on their cars. Even to this day, the GT-R-supporting areas of Nissan Motor Co. are still somewhat autonomous.
That's why you still see cars with the PMC.S stickers on them ( especially on the rear quarters of C10 and C110 Skylines ).