Hello L Cooper,
From their design and dimensions*, it is clear that the ‘curious indents’ you mention are merely another pair of the puzzling slots/holes seen in the Grand Gallery’s ramps.
Why this pair is at the base of the Grand Gallery’s south wall and not – as one might reasonably expect to find it – in the top of the ramps somewhere in the space between the 27th slot/hole and the face of the Great Step is a bit of a mystery.
I suspect that there was a change to the original design of the Grand Gallery – say, the south wall of the Gallery originally being at the end of the actual Gallery floor, and then being moved southward to create the Great Step.
MJT
* Details first recorded by C. Piazzi Smyth. 1867.
