Identify this piece please? 2x2 plate with 8-sided bar running around it
2x2 Modified Plate with Octagonal Bar Frame, also called Parabolic Ring. Part number 75937.

Older version: http://www.bricklink.com/v2/catalog/catalogitem.page?P=30033#T=C

Slightly different newer version: http://www.bricklink.com/v2/catalog/catalogitem.page?P=75937#T=C