The top part of the broken and fallen ‘fat’ upright from the central Trilithon 3 — it lays across the Altar Stone. Much of the graffiti on the top has worn away. Can just make out:
ƗANE FLEMƗNG 1747
I suspect the upper case letter I with the stroke through it would be the Latin form.
The colours of the lichen in the rain are emo grunge : -) And the ‘mineral iron’ in the stone adds ‘red’ staining to the puddles.
Much of the rounding has been done by ‘Victorian’ collectors with hammers.
But that cut, on the right… Maybe it was flawed when the builders were shaping it.
There are many eroded scars on the top of S55a, where some mineral has washed away over the millennia.