The tsuba is the handguard of a Japanese sword, such as a katana. It is a circular or sometimes oval-shaped metal guard that separates the blade from the handle and protects the hand of the wielder. While it is true that some historical katana may not have a tsuba, it is relatively rare.

The absence of a tsuba on a katana could be due to a few reasons:

1. Historical Variation: Throughout history, there have been various styles and designs of Japanese swords, and not all of them featured a tsuba. Some older blades or specific regional styles may not have included a tsuba as part of their design.
2. Functional Considerations: The tsuba primarily serves as a handguard, protecting the wielder's hand during combat by preventing it from sliding onto the blade. However, some blades were designed for specific purposes, such as cutting through armor, and may not have required a tsuba. These specialized swords might have had other forms of hand protection or none at all.
3. Personal Preference: In some cases, individual sword owners or craftsmen might have chosen not to include a tsuba for aesthetic reasons or personal preference. Swords were often customized and modified to suit the owner's taste or needs.

It's important to note that the presence or absence of a tsuba does not necessarily determine the authenticity or value of a katana. The design and features of a Japanese sword can vary based on historical context, regional traditions, and the preferences of the swordsmith or owner.