Each of the three Watts R13 Series RO Systems use proprietary (non-standard) RO membranes designed to fit special filter housings.  The R13 Series includes three separate RO systems:

R13-0250 System: uses a single 250 gallons per day RO membrane.  Manufacturer Part No. R96310 (Watts), described as a 3" x 10" element.

R13-0600 System: uses a single 600 gallons per day RO membrane.  Manufacturer Part No. R96320 (Watts or Alamo), described nominally as a 3.2" x 18" or a 3" x 20" element.  Also known by part number IHA-X00492.

R13-1200 System: uses two 600 gallons per day RO membranes.  Manufacturer Part No. R96320 (Watts or Alamo), described nominally as two 3.2" x 18" or two 3" x 20" elements.  Alternate Part No. IHA-X00492

Membranes for sale here are Watts originals, not the poor-fitting aftermarket replacements.