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Here are a couple shots of it. A friend of mine traded it to me for one of my Kuro slate "found" stones - he said the stamps were removed when he rounded the sides and edges - he does this to all his...

Maybe do the stropping first for the next photo, lol. I have a Nakayama Maruka Kiita with Nashiji that produces some very outstanding edges also - like yours it isn't ultra hard, but doesn't self...

It looks like you have a very slight folded edge or wire edge to me.