Nakajima Ki-201

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Interesting to know, thanks Jon. 
Might have to look for one or two.

They are not exactly cheap, but as Jon said, they are easier to clean than normal files.

Oh yes, I use an old toothbrush under the tap and it comes up like brand new every time. 

Also, for small parts I lay it on the table and gently drag them across, makes it easy to get nice flat surface planes. It's how I did the wing joints on the Thin Wing Javelin.