See the lowermost symbol - what are its origins?

Grave Mysteries

See the lowermost symbol - what are its origins?
Credit
Toby J
Taken
06/09/2018
Camera
Nikon D7100
ISO
320
Shutter
1200 second
Aperture
ƒ/5
Focal Length
16mm
Lens
Tokina AT-X 116 PRO DX II 11-16mm F2.8
License
Copyrighted
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A Numerical Mystery in a Welsh Churchyard