This Single Non-Pressurized Filterbasket from Ascaso measures 56.5mm and is designed for use with the Ascaso Dream, Arc, Basic, Elipse, and Uno Steel models.
Will not fit the Steel Duo or Uno Pro. Designed for the 57mm group head only.
A non-pressurized portafilter and basket combination generally yields better tasting espresso shots. It takes more work to perfect your tamp and less work for clean-up afterwards.