I keep seeing auctions for Nakayama Marukas that are all koppas. That can't be right, can it ?
Maruka koppa does exist. Various reliable sellers have sold them but like stated above be very careful what you buy and bid on. I believe it comes down to looks, cut, be it skin if you believe in that etc. Then there are different variants of the stamp itself. You put all those things together, throw in some faith in the seller and you buy.
One thing to keep in mind, a lot of razor stones were cut smaller than full size. Here is a pic of a hatanaka stone, they usually come with the maruka stamp as well.
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