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

![box extender][2]


  [1]: http://www.homedepot.com/h_d1/N-5yc1v/R-100149592/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=box%20extender&storeId=10051
  [2]: https://i.sstatic.net/86O7Q.jpg