The source is the same for all the charts. The daily newspaper VG (Verdens gang). They started publishing their own singles chart in 1958, and has been considered the official Norwegian chart ever since. In the 70s and 80s there was also a Top 15 singles chart monitoring the sales from record labels to stores published in Dagbladet. This chart was also highly regarded, and produced some #1's that didn't even chart in the Top 10 of VG-lista. Sadly it's not available anywhere on the net.