This filter is for the B301 series air pumps and fits only earlier versions of those pumps. Usually made before 2006. If your filter has 3/4" filter holder on the bottom you need the smaller filter which is the newer air filter setup.
This same bottom filter is used in the more modern Rena Air series pumps also. Item number: B3210BE
Description: Air Filters (2 ea) for Rena Air Series & Rena 301Air Pumps
Use the search box at the upper right of most pages to easily find this filter.
Direct Link: https://www.renaaquaticsupply.com/Air-Filters-2-ea-for-Rena-Air-Series-Rena-301Air-Pumps_p_696.html