Signals at Trowse

[Augustus.]

At Trowse on the GEML some of the signal ID plates in addition to the signal ID there is a sequence of letters and numbers on the base of the plate.
The text was smaller than the signal number displayed on the ID plate.

Does anyone know what this additional information relates to please. I think one of the signals in question was numbered 559.
Thank you.

 

[Shwam3]

They are OS grid references, CO559 has TG 244 073 as seen in this video:

 

[Augustus.]

Thank you for the information.