Question: What is the “B” or “P” I sometimes see on the white dragon (soap) tiles?

Answer: The “B” or “P” which can be found on the White Dragon tile, can be found in some Mah Jongg sets. These letters represents the Chinese Character Bai or Pai (pronunciation varies depending on the dialect) and translates to adjectives such as white, snowy, and blank. It is also referred to as Pai Pan meaning, white board.

Typically, you’ll find these letters in the middle of a simple or decorative border. However, in some older sets, they might appear as a standalone letter in the center or a corner of the tile. Apart from designating the tile as a blank tile, these letters hold no significance in the American version of the game.

Originally, White Dragons were blank tiles with one side featuring bamboo and the other bone. Over time, they transformed to being crafted from a single-colored plastic. This made it much harder to distinguish them as White Dragons. To address this, a rectangle was added to the tile’s design to clearly identify it as a dragon. Eventually, the term ‘Soap’ was embraced as an appropriate moniker for the White Dragon, as American players perceived this small blank tile to resemble a bar of soap.”

Sources:  https://www.themahjongtileset.co.uk/
Mah Jongg history expert Tony Watson.