Anyone know what scale this is for?

From what I can see, there's no real scale associated with this tool. It looks like it simply punches 0.25mm discs from plastic card which can then be glued to wherever they're needed.

So in 1/35 scale it would create "rivets" of 8.75mm, 1/48th scale would be 12mm, 1/72 scale would be 18mm. It all depends on what you want to use it for. 

Hmm...

Perhaps I was wrong about how this is supposed to work. I've found a review of this too which suggests it does indeed work in a similar way to what you have described, ie. punch indented rivets in situ on the model's surface.

http://www.ipmsusa2.org/reviews/Products/hasegawa_rivet-stamper/hasegawa_rivet-stamper.htm

Well, that's interesting. I'm not sure about the utility of a tool that has to be hammered to produce "rivets" though. This would obviously be highly impractical in some applications, such as curved surfaces...seems to me you would have to vise the part or otherwise find a way to hold it immobile while tapping with a hammer. All that would seem to increase the likelihood of damaging the target part. For flat parts such as wings it might be easier. Still, I'm a little leery about the idea of having to use a hammer, even a jeweller's hammer such as the one I use with my punch and die sets.