This is the rimless version of the 5.6x50R Magnum developed by Günter Frères in 1970 and it was introduced by DWM in 1971 with cases with the K T date stamp.  It was less popular than the 5.6x50R Magnum upon which it was based and production by DWM ceased in 1972 and this was the last cartridge developed by DWM.  RWS produced the cartridge from 1972 until 1977 and was listed again by RWS in 2004.  Hirtenberger and Impala bullets have also produced the cartridge.
The 7mm GJW is based on this case.