冑金 Kabuto-gane / Helmet Metal

What is Kabuto-gane in Japanese Katana terminology?

This refers to the metal fittings applied to protect the handle head (Tsukagashira). Basically it's Kashira in Uchigatana Koshirae, just a different name in Tachi Koshirae. 

During the Heian period, it was attached to the ""Kazari Tachi"" (decorative long sword) worn by court nobles during ceremonies. It often features luxurious carvings and inlays, emphasizing its decorative nature. The name Kabuto gane is because it looks similar to a kabuto (helmet)