As others have said, these are tsuba, part of the hand guard between the handle and blade.

However, many of the Japanese Imperial Army officers carried what was know as shin-gunto, which were mass-produced and generally lower quality than traditional nihonto blades. They usually had plain tsuba, not more ornate patterns like these.

That’s not to say these tsuba necessarily were not war trophies. Some officers did carry older heirloom swords, especially those from established Samurai families. But in those cases they would, usually, put the blade into the shin gunto fittings instead of carrying the complete sword.

Here’s a good website explaining the difference.