One New tire and square 0.9mm belt for Sony walkman model WM-D6C
This tire has one side slanted just as the original tire is slanted because the tip of the motor shaft is conical in shape. Typically what is sold online is tires with both sides flat which are not as the original one which has one side slanted for proper contact with the conical shaped tip of the motor shaft.
 
I use this tire in the restored walkmen that I sell, so it does work very well indeed. Replacing the square belt with a new one is also necessary, otherwise the autostop might not work properly. If you need a belt for the tape counter too, please get in touch.

Please remember after installation to clean with a q-tip dipped in alcohol the surface of the tire that contacts the motor cone to avoid slipping over the tire during playback.