I need to run down some pix of that and make it available, I do have some. They aren't hollow. weigh about 55 lbs each, are slightly wedge shaped, narrower at the back. The back face is sloped, and has about a 1x1" hangdown so they can't slide forward as it hooks over the back edge of the one below. Std landscaping block, but bigger & heavier. When I made the mold, I made it 6 blocks long and it split lengthwise to allow the finished blocks to be removed. Starting out with 3/4" plywood for the top & bottom surfaces, I quickly found I would need to stiffen it up as it warped, so it got a grid of 1.5" square steel tubing welded up and bolted to it, and lined with alu flashing to try to keep the moisture away from the plywood. I also put wheels & a tongue on it so I could wheel it around, putting it under the carport roof while they got a 1 day cure before opening it up and stacking them in front of the house for use. There are around 300 of them. Actually they got maybe an 18 hour cure time as it took me about 6 hours to open the mold, remove 6 blocks, clean and re-grease the mold, screw it back together with about 60 3.5" deck screws, run two loads of mix in a $190 HF electric mixer, pour it and place the decorator rocks in the face up blocks. That was, as far as I was concerned, a full days work and definitely beer-thirty by the time I was done. The steel grid work was a good excuse to make some legs to hold it a little higher when it was tipped over to open it up, so one side has a pair of legs welded on, cross braced at the bottom so I could roll that 400+ lb puppy over. Much easier on the ancient back that way. I've made a link to that dir on my web page, but haven't yet run jigl on it, brb. Done. The gizmo with the handle laying above & right of that coil of garden hose in the last pix, is a block carrier that works by gripping the front and back faces of the still tender block so I can lift it out of the open molds pocket. You can click on a thumbnail several times, eventually getting to the huge, right out of the camera images.
I've been needing to do that for quite a while, but these pix predate my building the web page. Thanks for the nudge. But I need to start specing colors I guess, FF5's choices suck dead toads thru soda straws. Gotta read up some more on how to do that in html. ;-)