Because OmniFilter has updated their housing designs, they are selling this kit with o-rings that fit the new and old systems to ensure customers get the correct o-ring for their system. This is why the K4 is listed as fitting the same housings as the OK30, for instance.
O-Ring Set Includes:
(1) Blue K4 O-ring for OB3, SFM2 Series A, CBF1, CBF3 Series A, OT32 Series A, RO2000, R14, CBF20, & OB1
(1) Black OK30 O-ring for CBF1 Series A, CBF3 Series B, OT32 Series B, SFM2 Series B, RO2000 Series B
(1) Black OK40 O-ring for OB1 Series A, OB5 Series A, R12 Series A, U25 Series E
(1) Black WC9–13 O-ring for OB5 w/white Head & OCT2
DOES NOT FIT: U24, U25, U26 & BF7

Replace your O-Ring with every 3rd filter change to prevent leaks
OmniFilter Part Number: K4

PLEASE NOTE: If your system requires 2 of the same O-rings, you will need to order quantity 2 of these sets