I have a few of these and so far the “leather” grip has held up quite well on them. Whatever it is, it’s not cheap tacky stuff.
Edit: I translated the text on the catalog and it says cowhide (牛皮). So it is real leather.
Another interesting thing to note is that the 5000 yen models have a shorter lead sleeve. When I got the first one like that, I thought the tip was damaged and pressed in, but then I found a couple more with identical tips. The catalog photo confirms that the tip is shorter 