What is it? These are the spline keys used to sync the rear hubs and the axle.
Why we love it? When doing maintenance on the rear axle, it is very easy to lose or bend out of shape. Having extra of these is vital.
Pro Tip! If a rear wheel hub with the wheel key does not slide onto the axle smoothly, there's something wrong. This is likely due to the wheel key being bent. On that note, if you have to force anything when installing on a pedicab, something is probably wrong.
What are the options? These come in pairs.