Mystery Object Help?

[BBP]

http://neco.aki.gs/neco99/neco30/02/sakura/s9011.html
A few days ago I found this charming little object in a charity shop, and after a day's deliberation I dipped out the 1,50 it cost. Thing is I don't know what it is.

There's a sticker at the bottom with instructions how to use it - of course in Japanese. I can order beer, discuss music and say there's a purple elephant in my garden in Japanese, but I read at the level of an 8-year-old, and using stroke order based dictionaries, that's not working. I could see the word "rensu", lense, but that's about it.

After much searching I found the brand name is Sakura - besides the Japanese word for cherry blossom (googling it gets you lots of pretty pictures) it was apparently a camera related business; the page I linked to is the only source of photos of the item I could find on the WWW. Google Translate gives something about camera focus aid.

Anybody here know what it is?

[Collector]

Sakura focus measuring instrument, at least according to Google Translate.

[BBP]

I gathered that, but that doesn't really give me an idea of what it does or how it's used. The diagonal pane is tinted, the top pane is glass, and in the box there's a diagonal mirror.