The No4 type modified fore-ends for the No5's we used to get through Ordnance could be identified by the square wood patch inserted into the formerly open lightening groove of the No4 fore-end. Over timne this was almost invisible. Same as the converted No4 butts. The little insert for the sling swivel plate was there but barely visible.
It is the most simple job to convert No4 butts, f/ends and handguards to No5 spec