interesting how that works huh? you can read all about how they developed the slo rainfly for us on the AT website,,, but they didnt develop squat.
I just took a look at the AT blurb on the SLO fly, it looks like they just copied much the description from the SLO website but left off the "SLO Sail and Canvas is offering custom-made FlipPac rain flies. " and removed other references to SLO that are on the SLO website. They probably just feel that is salesmanship but it may not sit entirely well with some. When I ordered my flippac FRP made it a point to ask me if I wanted a version with side windows or a version without side windows but with thicker fiberglass (same price for either) and also made it a point to tell me that the AT version was exactly the same.
I've had the standard widow thinner fiberglass flippac on and off the truck several times and driven some pretty rough roads without any problems so maybe the wall thickness isn't that much of an issue. Maybe without the windows the thinner walls would drum or vibrate so that may be why they use thicker fiberglass with the windowless version.