http://www.unicelfilters.com/pdf/replacement_guide.pdf, page 46:
Mytilus FMY 150 150 7-7/8" 33-3/8" 17-4980 C-7679 4 oz
Note: Product can also be replaced with one C-7699 and one C-7678.
Unicel C-7679 Replacement Filter Cartridge for 150 Gpm Pac-fab @ Amazon
Don't buy, till you've verified cartridge dimensions. You'll get best performance and life if you buy TWO and cycle them periodically. Filter cleaning guide here:
http://www.poolforum.com/pf2/showthread.php?18044