2.5" x 20" Standard Cartridge: a 1 Micron Melt Blown 'Poly" Cartridge typically fitted to trap fine levels of sediments/mud/rust as the second filter in a filter system.

The dark discoloration at the end is a visual indication of plugging. The cartridge darkens outwards from the smaller diameter center.  A further check is by removing the 3rd sump and turning on the water inlet - a visual indication / comparison with unfiltered water flow.

This is a  1 Micron melt blown 'depth type' cartridge that is very useful as a 'sacrificial cartridge' to prolong the effective life of a carbon cartridge.