It is a great piece and a rare item too. It deserves the best of care.
Lexol is good since it is water based but my preferred leather treatment will always be properly applied pure neatsfoot oil (NOT neatsfoot compounds), nothing else does the job as well. Others will disagree and that is fine, just use as little as possible no matter what you use - more is not better. Avoid anything with petroleum products, this include all "Pecards" products and mink oil, etc. They may be fine for work boots and modern made, user grade leather items but should not even be in the same room with antique leather items.
That being said, I agree with Pathfinder an original item that has reached this point is unrestorable and is best left as is. Use only minimal or no treatments, they can not bring back life to a "dead" piece of leather when it reaches this stage. Keep it out of direct sunlight and keep humidity in the storage area at a level where you are comfortable, about 40% to no more than 60%. Do not "excersize" the leather. Undisturbed and carefully kept, this leather piece will still be around for viewing and research long after we are gone.