If you can measure the overall length it might help eliminate some aircraft/narrow down the possibilities. I can not see the actual bottom of the adaptor in the photo, but I assume it is touching the wood base? For example, if the length is about 5' 1" or less it would not be from a Typhoon or Tempest, but could be from a Spitfire.
Otherwise, if you have not already done so, look for more stamp marks on the adaptor. There should be more stampings on the adaptor, either in the same section/area as the RA13718, or on the boss where the F H M inspection stamps are.