Complex sentence: More recently, Sturm Ruger and Company introduced a number of new lever-action designs in the 1990s, unusual because most lever action designs date from before World War II, in the period before reliable semi-automatic rifles became widely available.
Split into simpler sentences: More recently, Sturm Ruger and Company introduced a number of new lever-action designs in the 1990s. That is unusual in that most lever action designs date back to before World War II, in the period before reliable semi-automatic rifles became widely available.