I got the end dollies to which you refer from Schaff, I think. You might consider getting Pianotek's catalogue - their # is 1800-347-3854, or 248-545-1599. They have a new center dolly made of metal that is tippable at either end. Seems like the wheels in the middle were bigger, enabling the wheels at other end to tip. Very sturdy looking, nicely appointed so as not to scratch the piano. Basic question, though, is what someone wants to do with a dolly. That determines what kind to buy. There's everything from the center dolly to end dollies to Rosco, which is a motorized stair climber mfg by Arbogast's in South Charleston, WV. Good luck.
This PTG archive page provided courtesy of Moy Piano Service, LLC