Complex sentence: The manuscripts represent the "Quenis Majesties juelles plate and other stuff" in 1574 and additions by gift or purchase over the next twenty years which were kept in the Jewel House at the Tower of London.
Split into simpler sentences: The manuscripts represent the "Quenis Majesties juelles plate and other stuff" in 1574 and additions over the next twenty years. The jewels were kept in the Jewel House at the Tower of London.