I feel your bead may have been intended for usage as an ojime. Clues:

1. The bead is Japanese. We assume this because of the signature. Ogata Kenzan (尾形 乾山, 1663–1743), originally Ogata Shinsei (尾形 深省), and also known by the pseudonym Shisui, was a Japanese potter and painter.

2. There is no wear around the perforation at the top. This means, like an ojime, it was not rubbing against anything abrasive.

3. The hole is small but the bead is small. And, two extremely small cords may have worked on a lady's or child's ensemble.

4. It is in unusually good condition for an eighteenth century bead. The finest beads are often in the finest condition. Because they usually exchanged hands between a connoisseur or another proper custodian.

5. If a Kenzan, this bead would have been earlier than when most ensembles were in common usage. So, I suspect it may have been made later than Kenzan's lifetime. However, netsuke/ojime/inro were being used during Kenzan's lifetime. We know this because of the "Soken Kisho," the first publication in Japan concerning netsuke, ojime, and inro, which was published in 1781 -thirty eight years after Kenzan's death.

6. I can only imagine it may have been commissioned by one of many wealthy Edo merchant class families in Japan when the idea of wearing netsuke/ojime/inro was newly in fashion. Or, Kenzan may have made it for his wife or child. He was born into the wealthy merchant class. And, this would have been an appropriate gift for a wife who collected Kenzan. -or a wealthy woman who would appreciate a netsuke/ojime/inro ensemble paying homage to Japan's most celebrated ceramicist.

I feel it should be gently removed from it's mounting and carefully protected in a padded box for further examination by a Kenzan expert. It belongs in a museum. Or Antiques Roadshow, at least. The Antique's Roadshow logo is "Hidden Treasures." And this certainly is a hidden treasure.

Best regards, Frederick