Basicaly, this all stems from the likes of Browning .30" Cal, MG34 & 42's, Etc Etc. IE: the barrels WERE Pinned in place & welded to spec.
HOWEVER, it was discovered that SOME People found that if you unscrewed the UNwelded Flash eleim off one of THESE variants of Deact. IF, you dropped it HARD on the muzzle of the barrel. This would shear the retaining pin & the Barrel would be VERY easily removed form the reciever!

To counter this, the LONDON Proof Master insisted that Flash Elims would be welded/pinned in situ. To prevent thier removal. Now somewhere down the line the words 'Flash Hider MUST be pinned or Welded to the reciever'. Has gotten a little bit confused. & the wording has been 'Generalised & sort of accepted into ALL Deact specs from then on!

At the end of the Day, it is THE Proofmaster who has the last say in anything connected with passing a deact. If he INSISTs on a flash elim being welded on. You Will NOT[/ get it passed unless this is done! It's HIS way, Or NO way. Unfair? yes. But His word is FINAL. HE has the last word, end of!