Quote Originally Posted by Peter Laidlericon View Post
Marking the rifle number on the top of the butt as in this case wouldn't make any sense at all because our armoury racks line the rifles up with the top of the rifles against the wall and then they're chained in along, through the trigger guards.
If it was with a unit any time after about 1987 then it would have been shoved horizontally in a rack designed for SA80 - which would be a good reason to have the number in a visible location on the top of the butt.

Another possibility is that if the rifle has a target rail fitted, then it would have been used on a firing point with a a small target bipod/stand fitted. Schools and units with a number of identical rifles do tend to paint or stamp identifying marks on them to differentiate the rifles on the firing point.