Shaun Whitehead expanded on this question in the Em Hotep interview:
At a first glance, the second blocking stone just seems to be a large, relatively rough-hewn block sitting on the end of the U-block. However, the intriguing thing is that there appear to be red ochre mason’s marks on both walls at the far end where they meet the second blocking stone. This may suggest that this U-block has been cut back at this point, to form ledges on both sides. It’s possible to follow this reasoning to several logical conclusions, all currently speculation.
The other question which lies within the scope of Djedi’s capabilities is: how thick is the second blocking stone? If it is, like the first “door”, more of a slab than a block, and if it is either inserted into the shaft or rests on a ledge, then this could be a pretty good indicator that there is something on the other side than the core material of the pyramid. Again, Shaun explained the difficulties involved in taking this measurement in the Em Hotep interview:
The intention is to try to determine the thickness of the second blocking stone with the miniature “Sonic Surveyor” that I mentioned earlier. This uses a similar device to that used by Pyramid Rover, however it’s much, much more difficult to build. Pyramid Rover’s device was so large that it covered about half the area of the blocking stone. Ours has to fit through the existing hole in the first blocking stone, so can be no larger in diameter than a pen. This includes the actuator for “tapping” the stone, the sensor for listening to the response and the electronics to process the signal. We also have a much rougher stone to try to evaluate. It’s really tough to get it just right. The development was somewhat hindered by the sad death of Ron Grieve.
The Djedi team remains confident that these difficulties will be breached, and that when work in the Queen’s Chamber resumes, modifications currently being made to the robot crawler will allow them to get a good read of the thickness of the second blocking stone. Until then the questions remain—if the shafts ends with the chamber, and the first blocking stone was simply the dressed facing stone for the end of the shaft, why was it inserted 190 mm down into the shaft rather than flush against the end of the final u-block, and, if the chamber is not the end of the shaft, why is the second blocking stone made of the rougher, unfinished limestone, rather than the dressed Tura limestone?
These questions will only be answered, if ever, with further analysis of the shaft and the chamber.
Other Marks Inside QCS
When the Djedi team was able to analyze the video footage from the southern shaft they made another interesting discovery. On the lower right hand wall of the shaft, about 3.5 meters before the first blocking stone, there are some additional marks similar to the hieratic glyphs discovered inside the chamber. Like the other glyphs, these marks appear to be made in red ochre and black paint. According to the official report, the marks are about 3-4 cm tall, but “as the marks were found serendipitously, it was not possible to examine them closely” (p. 214).