These molds are made by the Ceramic Shop:
"If you have ever struggled with adding a foot ring to your hump-mold-formed functional work, you will love our Hydro-Bat hump molds. Made out of the same durable, absorbent plaster as our standard Hydro-Bats, our Hydro-Bat hump molds stand out from the crowd because you can fit them directly on any standard pottery wheel head and throw a foot ring or an entire form with ease! Our Hydro-Bat hump molds feature two rubber grommets cast into the underside of the form, set 10" apart, for fitting on your wheel head. After placing the hump mold on your wheel, simply lay a slab on top of the hump mold. Next, you make a foot in one of two easy ways - by gathering clay from the slab’s center while your wheel is in motion, or by attaching a coil and compressing it to the back. Please note that you can still use these forms as regular hump molds if you don’t have a wheel! Hydro-Bat hump molds allow beginners and professionals to consistently make larger slab-based forms with ease and speed." -The Ceramic Shop
You can also view their video demonstration using the molds here.