The top of the action i would say has not been filed off, at the start of the original radius for the flat is some stamp so it has been filed down a few thou, plus being a No 1 Mk V action why put an old pre SMLE barrel on? i can only think that the original was removed long time ago maybe it was a heavy barrel like the Lithgowmade rifles had, and this was put on to replace the original and the raised part of the barrel was lowered to match the action ?