This is one of two caches hidden to commemorate the airmen who died when their Mosquito aircraft crashed on Aran Fawddwy on 9th February 1944. More info can be found here
This cache is hidden alongside the propellor of the aircraft, while it's partner cache (LR412 #1) is hidden at the base of the mountain alongside the aircrafts engine.
You're looking for a small tupperware box, tucked away from the elements.