*** Final updated as of 01 AUG 2022 ***
The initial coords will take you to WP1, where you will be required to take note of the year the plaque was unveiled.
If the year was 'ABCD', you need to make the following calculations;
South = s31 50.441+D
East = e115 48.309-[(B*D) - (C*10)]
The new coords will give you the location of the hidden cache which is a preform tube. Happy hunting