No.. RG was producing a variety of headstamps at one time such as RORG. It is not a particularly difficult technical job to do this.. all you do is change the heading die, which have to be changed fairly regularly anyway as they are subject to wear.Although "RG" is Radway Green, did they actually manufacture it? I wouldn't have thought there was sufficient demand in the UK to warrant a dedicated production line. RG have certainly sub- contracted others to make ammunition - https://forum.cartridgecollectors.org/t/rg-04-9mmx19-jacketed-soft-point/60964. There's no trace of "TUN" in the 2000 and 2013 Monograms list. The lack of NATO standard symbol might suggest civilian consumption. However, a number of NATO countries, ourselves included have used .308 for sniper rifles - has there ever been a .308 NATO standard?
TimG