What material are those molds made out of? Positioned on the pallet makes it look heavy
Note the two primary feeder tubes protruding from the foreground mold used to vacuum the air from the mold itself during the process.
The manufacturing process works on vacuforming a raw plastic substrate which is heated and then a series of tubes on the underside essentially draws the air out and forms the plastic to the shape of the mold itself so there is no need for a second mold half.
I have also attached a pic of the underside of another original BW ramp mold showing the hollow underside and the series of tubes I mentioned as part of the vacuforming process (again all these pics are courtesy of PinballInc.)
I have many more photos of original BW ramp molds if you are interested. 
Thanks for posting the photo's! I haven't seen those in a couple years. As for the "feeder tubes" those are actually water lines used to cool the tooling between "pulls". When the tooling is used there is a wood bottom (not shown) covering that entire bottom cavity. The wood has a big hole with a flange which the vacuum line is hooked up. Sometimes when the tool has a really big cavity on the bottom (usually from parts which are "tall" and require a taller tool) the cavity will be filled with plastic balls to help fill the space and provide a better vacuum when you have a lower vacuum capacity.
