The first edition of Bitoku contains 2 promo's: one for Bitoku itself and a second one for Silk.
It's an improvement tile that will replace one of the original ones, as you can still only have 4 in a game.
Rules:
Each time the ookami is able to send three or more silkworms to her cave in the same move, the player whose turn it is may decide, in addition, to place a silkworm of their own color on the Shin Ookami tile, either from their reserve or from the Ookami’s den.
At the end of the game, during the final scoring, the player with the most silkworms on the Shin Ookami will get 5 points +1 additional point for each silkworm of their color on that tile.
If a player has no silkworms left in their reserve, they can use the “Raise a Silkworm” action to get a silkworm back from the Shin Ookami tile and return it to their own reserve (just like silkworms are rescued from the Ookami’s den).