Man, if I was going to pick up a W140, I would have picked a 6 cylinder, they are WAY lighter
The keys look familiar to me, I was working in a dealership back when this would have been new. It's just a barrel key, could have been for lots of different stuff. Is there a brand on the key head?
What comes to mind first are aftermarket wheels. They had these fairly poplular wheels that were chrome and had a large circular disc that covered the wheel bolt area, typically with a gold MB star on it, and the cover was held on with a single bolt with a head that used this type of key.
I also saw a car that came in with an aftermarket lock over the hood release lever that I believe used a barrel key as well.
Last thought would be a hidden storage compartment, maybe you haven't even found it yet! Check around the spare tire well.
Good car!