You'll need to complete the other three caches in the Old Mountain Road to Brynaman series before you can get the coordinates for this cache. Clues to this cache will be provided on the laminate inserts in these caches.
To get the coordinates for this cache, plug in the numbers to the following:
N51°4B.RYP
W003°5D.K(A+D)Y
Temporary (15yrs) error correction: B is NOT 8. Repeat - B is not 8! You'll know what is meant if you have completed the other 3. Just a clarification of something that would be very obvious by the time you got this far anyway. Thanks to Satellite Kid for pointing out the problem.
Some advice: If you're completing this as a series, have transportation parked in the car park way back on the other side and are going to have to walk back the way you came, you may wish to double back at cache 3 (Circle of Stones) and drive to this location as the next stretch down to Brynaman is by far the boggiest.
Alternatively, you may wish to do a 2 car trip as EP did - park one car each end (with a friend joining him to do the walk in his car). It's not as carbon friendly but it halves the leg work!
A third way, if you have to split your mission for the weather, is to park at N51°49.757 W3°51.515 (near the new-fangled SPECS, solar array and generator - how did they get planning permission??) and strike west through the old car park to reach OMRtB3, do the boggy bit, pickup the bonus and then dry out on the return east of the river.
Whichever way, if you intend to park at this end of the track, EP suggested parking at N51°48.851 W3°52.038 and walking along the little paved footpath. A new bridge had "recently" been built to replace an old one so the river is now easily crossable once again, and just recently (2023) the deck has been replaced again.
Best wishes in finding the bonus. If you somehow fail to find it having found the other three and wish to see a spoiler image, please get in touch (include in your message the alphabetical letters you found in the three caches).
