The left side is MEANT to be higher than the other. It is a reminder of its ancestry when the first No4 prototypes had a magazine cut-off. It allows the cut-off to slide over. Never modified when the cut-off was deleted from No4 series production.
It was suggested at the time but the report into the comment stated that if both sides were made equal height, it would immediately mean that there would be no replacement magazines for those early and trial rifles in service. No, but someone should have answered back. Well take the bloody cut-offs off them then!
That crappy sanding appears on brand new UKproduction ones. Paul, just get on and fit the bloody thing as we've suggested. It should take 5 minutes to get that upo and running if you want my opinion as a bit part amateur