I recently mounted the 43mm SK model on 21mm internal UST rims. It was really challenging, but this thread ironically gave me the motivation to keep trying. I used two Pedros levers and quite a bit of force (though not enough to think anything was gonna break) to get them to cooperate. This was all to get the to the point of dry mounting the tires in preparation for going tubeless. I was not able to get the beads to seat using a floor pump and soapy water. It took a few tries with an Airshot and soapy water to get them to pop on. Then I had to break the bead (not difficult) and take it off one side to pour in the sealant. The second time remounting the tire post-sealant was considerably easier, but I still needed levers and some force. Tire/rim/sealant combo holds air really well. Experience was identical on both tires and rims. I'm gonna keep the two levers in my saddle bag from now on, just in case, knowing it's impossible (for me) to do anything with this tire/rim combo by hand.