This is a pack of ten 15x21x4mm High Speed “HS” from Ultimate Racing.
These 15x21x4mm bearings are intended for use with the Mugen buggy and truggy 1/8 scale vehicles. This bearing is the stock replacement bearing for the inner wheel hub and can also be used as a standard bearing for other RC vehicles. Please check the manual that came with your vehicle for the correct size if you are not certain.