Chris, if I understand what you're referring to right, that buckle is made for 3/4" webbing. It's the one on either side of the belt that lets you snug it against your back.
Thanks to Doug's idea, I had Mike do something similar on a 3400 Porter, except I had him shape the lumbar pad for the Granite Gear Crown hipbelt instead. It's also designed to find the Circiut/Ohm 2.0 hipbelt as well, but I haven't tried to fit it in since I don't have one anymore. I just wanted that option if I chose later.
FWIW, I've been pretty impressed with the Ice Pack belt that Mike sent with it. It's not as comfortable as the GG belt, and a couple ounces heavier, putting it about the same weight as the ULA belt. But it carries heavier loads (say +25 lbs.) better, so I see no reason yet to have the ULA around.
So far, I've been very pleased with the mod. Keep in mind that I've only taken it out a bit given the winter, but early test results are very positive.
Which Windrider do you have? The belt on my wife's 2400 Southwest is a lot less padded that the Porter models I've seen.
/*/Edited to fix webbing/buckle size/*/