Complex sentence: The original treaty is entitled the "Treaty recognising the sovereignty of Norway over the Archipelago of Spitsbergen" and refers to the entire archipelago as "Spitsbergen", which had been the only name in common usage since 1596 (with minor variations in spelling).
Split into simpler sentences: The original treaty is entitled the "Treaty recognising the sovereignty of Norway over the Archipelago of Spitsbergen". It refers to the entire archipelago as "Spitsbergen", which had been the only name in common usage since 1596 (with minor variations in spelling).