A bottomless portafilter is a diagnostic tool that allows a user to observe the espresso extraction process directly.
It lacks the spouts of a traditional portafilter, exposing the bottom of the filter basket.
By watching the shot being pulled, a user can identify issues with their grind size, tamping, or coffee distribution.
The portafilter in the image appears to be a 58mm size, which is a common standard for many professional and home espresso machines.