Convex kamisori strop

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I recently bought an Inoue Tōsuke NOS kamisori strop. It’s almost exactly the width of my Iwosaki at 48mm, and the leather seems to be of impeccable quality - a single strip, covering the length of the wood, one of the ends, and along the opposite, convex face.

What I’m puzzled about is the purpose of the convex face. I’ve speculated a fair bit and I’ve searched online, to no avail.

Does anyone know what its function is, or what advantage it provides?

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MarkP said:
No idea why its convex, is it to match the omote side of the kamisori?
I just use a travel sized wooden backed strop for mine.
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I did speculate about the omote side, considering only half of it is ground. I’ve always stropped evenly, though, and never heard otherwise.
 
would suspect you've got a defective piece there.

We have learned from the experts here and upon other similar venues that both the hone and strop are always supposed to be kept as flat as can be achieved.

This swordmaker's waterstones all seem similarly defective, so maybe it is problem over there with manufacturing?

 
I spoke to Tony Milton, who says he’s seen people use convex whetstones in Japan - for knives - but a convex strip was a new one on him, too. I knew it wasn’t defective - it’s a hand-made Inoue Tōsake item, initialled in gold pen on the end (I don’t know what the kanji are; they look similar to the Tōsake kanji on the box, but I’m far from an expert!).

The dealer simply said that the convex side was intended to provide a more delicate stropping effect, specifically for polishing the edges of kamisori blades. Considering the geometry, it made sense that the effect would be slightly different, but in the end I decided the only way to check was to test it out.

10 laps on stone, 40 on canvas, then 100 on the straight edge for shave 1; the exact same for shave 2 except the 100 laps on leather were divided 50/50 between the flat and the convex strop.

The results? They both have blindingly good shaves - one pass, followed by a partial pass along the neck area. On shave 1 I did give the blade 10 laps on canvas after the initial pass, whereas I didn’t feel the need on shave 2. Tbh, the difference is negligible at most, but one thing is clear - it’s a perfect strop for a kamisori user.
 
I was joking; the strop is very clearly made that way on purpose. Japanese are very anal.

As a razor's bevels are supposed to be concave [according to any 19th c. grinders textbook I can find], it makes sense one would use a wheel-shaped surface (or an extremely fine wheel, though no such animal exists fine enough in the world now, the Tormek system's finest still not fine enough & hard Arkansas or Thuringian slate wheels now long gone from production) to address them, so that they would not thicken, thickness here being the enemy.
 
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