I've found another strange mystery...

"....one of the tallest (Bluestones) still retains a longitudinal groove down its
whole length, and only a year or two ago Mr. Robert Newall proved that
one stump below ground possessed a longitudinal groove right down one
side, neither of which fitted each other but proving nevertheless that
they had formed part of some earlier monument."
Stonehenge in the light of Modern Research by J.F.S. Stone 1953.
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