Non-hex socket wrench for Galtmaster

Hi, I'm trying to remove a recessed Galtmaster cartridge in a bathtub. Here is a link to it:

http://www.homedepot.ca/product/galt-tub-shower-hot-cold-cartridge-70283/907044

As you can see, the nut is not hex shaped; it has 2 flat sides, and 2 rounded sides. Where would I get a socket wrench for this (as it's recessed)? I don't even know what it's called so I can search for it on the Internet. All I get is hex shaped socket wrenches.

Thanks