This Fuchi Kashira represents a Kabuto, a samurai helmet seen from above.

• Material: Brass, silver Ibushi finish
• Fuchi (縁) Dimensions: 40.5 mm long, 24 mm wide, 11 mm high
• Kashira (頭) Dimensions: 36.5 mm long, 20.5 mm wide, 10.5 mm high

The name "Kabuto Hachi" (兜鉢) refers to the main body of the samurai helmet, which is made in a hemispherical shape to fit the shape of the head.
Iron was used mostly, but leather could sometimes be taken.
More specifically, this is a "Sujikabuto", a type of helmet where the rivets that hold the iron plate of the helmet bowl (Hachi) are not visible, and one side of the iron plate is bent backwards to give a muscular appearance.
This type of Kabuto was made from the Nanbokucho period and became common in the Edo period.
Also, during this period, bladed weapons such as swords and spears were popular in battles, so it is believed that the helmet was designed to be slippery against blows to soften the impact.