This appears to be a 1/2 inch [extender][1] that fits octagonal boxes:

![box extender][2]


  [1]: https://www.homedepot.com/p/Carlon-4-in-Blue-Nonmetallic-Round-Plaster-Mud-Ring-for-Electrical-Ceiling-Box-A471RR/100149592
  [2]: https://i.sstatic.net/50Lh7.jpg "box extender"