Hey, it's parts for a "build-up" bicycle model, part of the "Pedal Mafia" Pedal ID series from Yamato! These highly detailed bikes can be disassembled and reassembled, and customized with option parts like this (all sold separately)! To see more at the Pedal ID website, click HERE.
This is a saddle set, as shown below.
See the Pedal Mafia Pedal ID bikes and all the cool option parts HERE!