Hi laffouine,
The L2A2 7.62mm Ball round was the standard British Military round from the mid-1950s up to the mid-1990s and beyond, and used in both rifles and machine guns. It had to have a particular standard of accuracy to be accepted for military service, but this standard was not particularly high.
Therefore any batch, which on testing for accuracy, was found to be very good (or, at least much better than the usual production runs) was reserved for sniper use, and for competition shooting, and the boxes marked "SNIPER" as per your photo.
The black circle on the bottom left of the label was the usual mark for Ball ammunition; some of the boxes with competition-grade ammunition were marked with a green circle, and were also usually stamped either "TARGET AMN" or "COMPETITION", this ammunition usually being referred to as "Green Spot".
Hope this info helps.
Roger.