7-1981-01 is a black O'ring that fits is used in DEL Ozone units.  The previous o'ring was PN 7-1871-01 and was made from silicone.  The new 7-1981-01 is made from EPDM which is more reliable when used with ozone.

7-1981-01 is used in DEL Ozone's:

• Spa Solar Eclipse (SES-U, SES-UP-T)
  
• Solar Eclipse for Pools (SEC-50-01, SEC-100-26)
  
• And the new AOP25 and AOP40 units