Hello Graham !

>it was therefore blocked up in its place, with a stout board across the passage,
>to prevent it being started into a downward rush. It was a slab 20.6 thick, worked
>on both faces, and one end, but rough broken around the other three sides ;
> and as it lay flat on the floor,
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This should answer Anthony’s question about granite on a limestone ramp. As Petrie states it lay flat on the floor therefore it did not slide to the bottom by gravity. His fear of it sliding and placing the stout board would be normal but not needed in this case.

Thanks for providing other evidence that the plugs would not slide by gravity.

RLH