On this tag summarize what you know about the history of this key. Use two tags if you have to. If it is a less well known key, add the maker and date of manufacturer also. String tags are available at office supply stores. Besides preserving the stories for posterity, documented keys add a lot of interest to your collection.
Visitors like to hear stories concerning keys.
One interesting story begets another. Tell a few and you are likely to get one in return.