Complex sentence: The story is remarkable also for the inital reference to a tin dispatchbox, hidden in a bank, where Dr. Watson would keep the papers concerning some Holmes' cases with no good solution or no explanation at all.
Split into simpler sentences: The story is remarkable also for the inital reference to a tin dispatchbox, hidden in a bank and full of papers by Dr. Watson. Some of them are record of the cases with no good solution or no explanation at all.