Hi This is about 8 and 5/8 mm inner diameter of 18.64 size R58.57mm inner mm circumference
hope this help regs John John Cadby FGA DGA Director John CadbyJewellers Ltd @John_Cadby_FGA_DGA www.Cadby.co.uk
That would be size 8 5/8 or 18.61mm in diameter. Hope this helps.
Phillip ScottT technical Support & SalesRio Grande