When you get pieces of bonus stamps or legendary baskets, you can find them in the Piece Factory. So where is Piece Factory? Please follow these steps:
1. Tap your avatar on the top-left corner in your game lobby, and you'll get to the Personal Vault.

2. Tap the PIECE button at the second place of your INFORMATION page, and you'll get to the PIECE page.

3. You can check how many pieces you have so far. Also you can combine pieces into completed ones once you gather 100 pieces.