IKEA lampshade: replacement?

Some years ago we bought this lamp from a Canadian IKEA. I don't know the model name.

I want to replace the shade (one side is stained) but it has a weird fitting as shown here. It's weird because the spider fitting is deep, and it's held onto the frame by a 3/8" thumbscrew. (It looks like 3/8" and not 10mm.)

Does anybody know where I can get a shade like that, or what that fitting is called? It's different from all the other shades in our nearest IKEA, which have European-type UNO fittings.