The Location of the Altar Stone Engravings

The location of the enigmatic engravings on the Altar Stone shown in Richard Atkinson's 1958 photos that I found in the Historic England archives is a bit of a mystery. Without doubt they are on the Altar Stone at Stonehenge but the caption "Surface Of Altar Stone From North East" doesn't seem right. We have his photo of the North East side of the Altar Stone (below, last photo) exposed after excavation and it doesn't match. I thought it must be the eastern end of the stone but again there are problems. The photo shows the surface of the stone and a section of soil  butting up to it, indicated in pink in the diagram below. His excavation photo shows he cleared the end so there is not section of soil left.

However examining one of the Historic England photos of the engravings reveals in the top right corner a groove in the top surface of the stone, above where the soil section is. Circled in pink in the marked up diagram version (Due to copyright I can't reproduce the photo).

My own photo of the middle section of the Altar Stone surface is moss free and shows an identical groove on the south side as it goes under Stone 55b. Other photos from other sources also show this groove.

Atkinson's excavation photo shows he dug on the south side of the middle section of the Altar Stone and left a baulk of soil under 55b. A photo taken of that excavation "looking" north east would match the features indicated. The eroded top edge of the Altar Stone appears very similar to that in the photo as well.

I am convinced the location of the engravings is on the south side of the middle section of the Altar Stone.

UPDATE - Aha! I have found more photos where the caption seems to mix up North and South, if 156 is on the left and 55B on the right you are facing North not looking from it. 

"Centre Of Altar Stone From N.W. Stone 156 On Left Stone 55B On Right".https://historicengland.org.uk/images-books/photos/item/P50100 

"S.E. End Of Altar Stone From N.W. Stone 156 On Left Stone 55B On Right." https://historicengland.org.uk/images-books/photos/item/P50099 

I'm booking a visit to the archive to check them and others out.

Diagram showing the features in the photo of the Altar Stone above

(Photo itself is copyright)

Altar Stone middle section surface looking from North East 

My photo - free to copy, click to enlarge.

Rotated cropped copy with the groove highlighted to match the orientation of Atkinson's photo: