Complex sentence: A series of modifications followed that attempted to correct these problems, leading to the ultimate "S-4" model, but none of these proved fruitful and most work on the pinch concept ended around 1961.
Split into simpler sentences: A series of modifications followed which attempted to correct these problems, leading to the ultimate "S-4" model. However, none of these proved fruitful, and most work on the pinch concept ended around 1961.