Complex sentence: It became widely known as the Anti-Slavery Society, as had the earlier society, and to help distinguish it as a new, worlwide, anti-slavery organisation, the society always strictly dated its origins to 1839.
Split into simpler sentences: It became widely known as the Anti-Slavery Society, as had the earlier society. To help distinguish it as a new, worlwide, anti-slavery organisation, the society has always strictly dated its origins to 1839.