The question of the propellant usd in Britsh 7.62mm NATO is not easy to answer in brief, but I shall try.
The specification for Ball L2A2 calls for a 144 grn bullet with a charge of approximately 44 grns of NRN (Nobel’s Rifle Neonite) propellant which is a flake powder.
Ball L2A4 was similar except that the charge was about 44 grns of Ball powder. This was approved in 1970, made only by Radway Green and very little was produced. Ball powder, despite its advantages in manufacture never became standard for Uk manufactured 7.62mm.
All this applied up until the late 1980s when Royal Ordnance became a commercial organisation and eventually part of British Aerospace. Commercial imperatives now applied and all sorts of cross border contracts came into operstion.
The most important of these was that with Hirtenberger of Austria who made large quantities of 7.62mm ball and blank from around 1987. These were headstamped as “RG” and loaded with a slightly heavier bullet weighing about 146 grns (from three different dated specimens) and around 45 grns od ball powder. The headstamp format was the “universal” one of “RG”, date and Nato symbol which meant cases could be used for ball or tracer etc. However, despite the fact the these rounds were closer to the old L2A4 with ball powder, they were still packaged as L2A2.
Since the main purpose of the headstamp is to identify the manufacturer in the event of problems, the MoD wanted these contract cases changed so in the 1990s Hirtenberger started using “HP” on British contract 7.62mm Ball, blank and 9mm ball and drill rounds. Also, new “L” numbers started being allocated to any changes in manufacturer, propellant, primer or bullet. Examples from this period are L21A1 and L37A1 ball from FNM in Portugal.
Now, every minor change is allocated a new code, so the RG sniper/target ammunition can be found as L42A1, A2 and A3, the differences being powder and primer supplier.
Recently more and more contracts have been placed, the latest with CBC, and we are up to L47A1 in the numbering.
To return to your original question, the round you show is one of those manufactured by Hirtenberger with an RG headstamp.