"Porcelain" is just a just a way to explain this dial deep and special shine. It is really obvious when you see a white dial and a porcelain dial side by side.

Apparently it was only found at random in a early R serial Daytona (maybe some very early L as dieter stated)

Beautiful but very (very) fragile when it is manipulated , at least much too fragile by Rolex standard and maybe only a "prototype" tested in a pre serie and then deleted..

Cheers,

Eric