All bicycles in Japan are required to be registered. Just take your registration certificate or receipt and they will release your bike to you. If you don't have it handy, a close description and any conspicuous markings or stickers will be good enough.

"Mama-charis" all look the same here, there are hundreds of thousands of them, and most of them are the same silver color. These bikes don't come with locks because someone is afraid of getting their's stolen, but because it is very easy to take the wrong bike by mistake. Many people put colored stickers and such on their bikes so they can distinquish them from others.