Ko-katchushi
  
80mm x 79mm x 3mm at the rim
 
This is an old tsuba that called Ko-katchushi group, it means "old armour maker".
It is made of steel and has a file mark on the surface. The rim is thicker than the plate, made as pipe.
The surface condition looks very old, before 16t century.

Katchushi tsuba is a style of old tubas. It doesn't mean that the tsuba is actually made by armour maker. But the style is typical of that armour makers made.
The terms it can be called so are
1) made of good steel
2) the plate is simple and thin around 3mm, sometimes having small cut-outs
3) some excellent hammer work on the rim is necessary, turn back, pipe or so.
4) when it looks before 16the century, it is especially called Ko-katchushi.
 
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