The K&N Stubstack is designed to increase the airflow of carburetors by decreasing the restriction around the choke horn.
The stubstack fits inside the air cleaner housing and slides snugly down over the choke horn.
They're dyno proven to increase hp.
For Holley carbs:
For Edelbrock and some Carter carbs:
Back in the day when I used to use these, they were meant for carbs with the choke removed but didn't have the horn milled off. Today's design looks slightly different from what I used years ago, and appears that it can be used on carbs WITHOUT the choke removed. IDK for sure though, so I'd check with Holley first.