This is the bonus physical cache to the Crowle History Adventure Labs.
It is not at the posted coordinates, which are for the town centre car park.
Collect the bonus letters by visiting the 5 Adventure Labs locations, and then find this cache at
N53 35.(E-A)(D-C-B)(A+B)
W0 51.(C-B-B)(B+C-D)(D-C)
Please note: The AdLabs used to be sequential, I have now made them non-sequential, but would advise completing them in the order specified.