This is a bonus cache for the Adventure Labs Olofsfors Ironworks - West.
There is another Adventure Labs in the area, Olofsfors Ironworks - East (with bonus GC8R90C).
It's best to start with east (and that bonus) before doing west.
Once you've done all the steps, just add all the numbers together (A+B+C+D+E) and you'll get a 4-digit number.
Enter that into certitude to get the final coordinates.
You can validate your puzzle solution with certitude.