What are these? (not datum plates)

[_toommm_]

On the south WCML today, I’ve noticed a lot of these plates on the OHLE masts. Some yellow, some white. I can’t tell if it’s a cutout, or a black circle. What do they indicate please?

 

[pdeaves]

I saw that on railsigns.uk but I cannot remember what it is or which page it's on....

Edit: Here it is: https://www.railsigns.uk/sect28page2.html (item 28.15).

These 'sighting distance devices' were provided to assist staff working on the line in assessing sighting distances, and they have no relevance to train drivers.

 

[Boodiggy]

Its was for lookouts to calculate sighting distance. Not been used for years.

 

[_toommm_]

Thank you both! Never in a million years would I have guessed that.