Looks like this is what you are looking for:
Waterway Pro Clean 100 => Unicel C-8409
Unicel specs are 8 15/16" OD; 17 3/8" long;

The pressures you report are consistent with a collapsed filter. I'd recommend getting two cartridges and swapping them out whenever pressure climbs to 5 psi above initial. Once you get the cartridges, use this guide to cleaning them:
Cleaning-your-Cartridge-Filter-wiithout-Damaging-It!