I think it might be the Petrie granite stone, that prevented Prof Smyth from clearing the descending passage. Petries description of it is on page 28 of Pyramids and temples of Giza, The measurements of this block agree with the one outside by the entrance, Jon Bodsworth many years ago kindly sent me an image of this block, with measurements placed on it, that agree with Petrie's description; though Petrie only mentions a broken drill hole, the other two may have been filled with compacted debris and not noticed at the time.

In Edgars work, plate LXXXII, page 197, we can see the iron gate that maspero fixed on top of the Petrie stone. I have no idea when this stone was removed to the outside, but it appears to be the Petrie stone, as Petrie, says it is 32 inches wide to the broken sides, and 45 inches long to a broken end, these are the exact dimensions that Jon has put on his jmage. I still have the image if you require it.