This is a spider for BCD50 cranks such as TA SUNXCD.
 Installation reference article

 The chainring side is BCD 110
 Stainless steel, 4mm thick
 Laser cutting
 Buffed finish

 I think it can be used for a wide range of purposes, from touring bikes to old-school mountain bikes.

 *Crank and chainring are not included.

 Price including tax: ¥7590