HEPA Ultra Bags
They're the second one from left to right. They're the "HEPA Box Ultra Bag," which has the additional layer of paper ontop of the synthetic material. Unlike the generic, it has part of the lamination structure found on the European Ultra Bag where it has sections where if you shine a bright enough light at it, you'll notice that more light passes through a small double dashed line where it appears they thermally laminated the material. The material is a little more similiar to the soft synthetic mateiral on the generic instead of the hard and more crisp feeling material on the European Ultra Bag. Strangely, they're made in Belgium and not Germany.
I'll see if I can take a picture of the lamination pattern difference between the HEPA Box Ultra Bag and the European Ultra Bag. The generics don't have that and if I lightly rub the bag I can actually pull up the outer layer of material a little. The layer reminds me of the microlining material found in the paper bag, but with greater density on the fibers. On the HEPA Box Ultra Bags the material seems significantly different, in that it doesn't "lift up" and it is significantly thicker. It seems like the material is a single piece and not multiple layers.
I haven't had a chance to put one on given how long the bags last and I have 16 of the HEPA Box Ultra Bags (A set from the X4 box and a set from the C3.1 box), but once I do I'll update this topic and report on the airflow. Going from the multi-layer paper bag to the Ultra Bags (As well as the generic synthetics) dramatically increased flow, even against a fresh paper bag. Even better is that the flow has been staying fairly consistent, there's a small drop in the beginning but then it leveled out. I still can't stall the Dyson TFT tool by pushing it against the same spot of carpet (For consistency in testing) even with the small drop during initial usage and it still spins significantly faster than it would if it were being driven by the C3.1 with a multilayer paper bag. It feels like both the C3.1 and X4 received a significant motor upgrade at this point, to the point I want to say they have 2-3x the flow... Or at least 3x when I put the bag in initially and it drops to 2.5 to 2-ish after significant usage (To the point the bag has a good amount of accumuluated dirt towards the bottom).