Yea, the buckles are just decorative, the straps are stitched down until about 2 inches away on either side.
They're not just regular sliders, the center bar seems to be a separate piece and it looks pretty large, kind of like the knurled bar on the roll pin buckles.
I was thinking the same thing. I haven't found any buckles like them yet. They DO make 1 inch roll pin buckles, but the frame looks nothing like these. Right now, I'm just planning to use some rectangular frame sliders. But I'm keeping my eyes out for the proper buckle.