A short video of the stone can be seen here
If you look in the corner of the rock below you can see a raised portion of the rock that did not get sanded to shape. I've outlined one of the pictures in black so it is a little easier to see.
This is a piece of one of the "H" stones that was sticking out of the ground. There is a little flaw in the corner that's a little hard to see in the picture. If you see it in person it is a lot more obvious.
So looking at this from the chipping, sanding, and chiseling theory.
You can see that just beyond this flaw, the corners are sharp and crisp. If they were sanding or chiseling the side, back and bottom they would have had to sand around this imperfection so that the raised section does not cause the sanding block to also follow this pattern. If they did allow the sanding block to ride up on it, it would cause a subtle radius over the entire face of this flaw, but there was none so they would have gone out of their way to leave this flaw, actually making it more difficult to flatten the face of this stone. I held a straight edge up to this face right up beside the flaw and it was perfectly flat. So you have to ask, if they have the skills to make such perfect corners on all of the rocks that look molded but go out of their way to leave a flaw in this one that they would have to work around so they don’t distort the entire face the flaw is on, why would they do that? When sanding back and forth you don’t want to have to avoid any obstacles, making it harder to sand.
Once the part has been chiseled out somehow they would need to go in the with a flat stone and basically use it like sandpaper to get all of the ridges left behind due to the chisel marks. This would have to be done if the chisel method were used.
If you look at the bottom picture at the top right where the block has been chipped you can see that this rough area has as many hills as it does valleys in its texture, and has the look of something that has not had any attention put into it after the corner chipped off. If you look at the flawed area this is not the case. If it were chipped and left un-sanded it would have the same look to it as the above corner that was broken off. This supports the theory that it was molded against it, not chiseled out.
So looking at it from the molded theory.
It’s very simple, someone dropped the wooden plug that fits into this area and dented the corner. A dent into the plug that fits in this area leaves a raised section once its molded, and that’s exactly what this looks like. This leaves all the rest of the corners in perfect shape It also does not deform the sides due to the sanding block riding up on this flawed surface. The flawed area looks like it has the same texture as the sides of the block, maybe a little rougher due to what the plug fell on, not like it was avoided after chipping like texture of the broken part the top right corner of the block. See picture below.
Below you can also see a bunch of air pockets all over this stone much like you would see in a molded cement part if a vibrating table were not used. The bubbles are not seen in the part of the blocks that have been broken off, only on the flat surfaces of the block the cement would sit against the molding surface. This look is also on my first quick and dirty attempt of making the arrow rock.
Lets assume this was the piece of wood that was to be used to make the cavity in the above block.
It's always easier to make the negative of an inside corner on anything you plan to mold. You will notice nice crisp clean corners just like you see in the H stones.
Hers is the part after it was molded and all of the balsa was removed. Once again you can see the corners are all perfect like most of the "H" blocks at Puma Punku, other than the damaged corner. An inside corner on a stone like this is almost impossible to make perfectly, especially considering the time this was built. If they had the accuracy to do this to most of the stones why would they go out of their way to leave these flaws in some of the stones? This flaw would be even harder to work around than removing it while they were doing the rest of the corners, it just doesn't make sense. A slightly damaged plug would explain all of this very easily. Many of these stones show subtle signs like this, I just did not have the time there to document as many as I wanted.