Generally speaking, this commercial style headstamp was first made by Munitionsfabrik Thun as early as 1992 and its use continued and was expanded to many different calibers after January 1, 1995, when the factories of Thun, Altdorf and Wimmis became SM Schweizerische Munitionsunternehmung. In next years it remained in use after several designation changes, as follows: SM Schweizerische Munitionsunternehmung AG (1999-2001), RUAG Munition (2001-2002), RUAG Ammotec (2002-2008) and RUAG Ammotec AG (2008-2013). Also, it can be found in “Blaser” brand cartridges marketed by Blaser Jagdwaffen GmbH.
The earliest reference that I’m aware mentioning a .300 Win. Mag. with this headstamp dates from 1999, but it might be earlier.