Couldn't say for sure. Looking at the top plug in mine, I'd swear it's G1/4. Comparing it to another fitting, I'd swear it's G1/4. But I can't get a different fitting to actually screw into the res for love or money.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133
I used a 1/2"-20NF Tap & ran that through to open up the threads abit on the top of the res. I was able to fit a barb in it snugly. It won't be seeing pressure anyways as it's just a dump for one of my loops. If I decide to use it that is.