Complex sentence: The Rocket3s/Tridents were immediately labelled "superbikes" when introduced in the summer of 1968: an apt description, since they were the first modern, multi-cylinder production motorcycles and amongst the very fastest then available.
Split into simpler sentences: The Rocket3s/Tridents were immediately labelled "superbikes" when introduced in the summer of 1968. They were the first modern, multi-cylinder production motorcycles and amongst the very fastest then available.