Someone on Forums has the history on this, so we may hear from them. I'd guess that some form of bowl coating goes way back, because there was such a competition between pipe makers, when the volume of sales was at its height, when dozens of patent systems and variations bid for the consumer's dollar. That's where all these various stingers, extra chambers for moisture, vents for extra air in the airway, etc., came from. I'd guess bowl coatings on particular brands went back to the 1920's at least, but it's just a guess.