I didn't have that many mystery keys, so about two years ago I finally went around fitting them to places with keys. I labeled the useful ones and threw away the rest. No more mystery keys for me.
I too have many old keys now defunct but I cant bring myself to part with them so they are in a drawer out of sight. Amongst them are small keys for suitcases used for hols decades ago and the suitcases have gone to the suitcases home in the sky.
I think keys are a very dominant factor in most people's lives and can cause much hassle...infact they are a pesky commodity but we need them to protect ourselves
I have six old "skeleton keys" somewhat like the picture above that came from my grandmother's house. One of these days, I'm going to have them gold- or silver-plated and made into pendants for my sisters.