Vincent Bach was a rare combination of artist and engineer. He constantly changed his mouthpieces in search of the perfect design. It wasn't until the mid 1960's that he stopped and finally settled on more standardized design. It was around this time that the formalization of the 27 throat and backbore to cup letter (A=24, B=7, C= 10, etc.) created a standard for mouthpiece making that is still used today. These standards are now known as the Classic Series.
https://www.bachbrass.com/instruments/mouthpieces/bach-classic-series-mouthpiece