Green key

We tore down our old back porch a couple of weeks ago. The job went well; destruction is so much fun. Actually more fun than construction, I think.

The find of the day was found by my son. After we had the decking removed and tossed into the dumpster he noticed a key hanging from a nail at the end of the porch near the basement door, but on the inside of the last joist so that it would be under the flooring. The key was green, so we assume it is copper. But the real questions are, how long had it been there, and what was it for? 

We've had the house since 1981, so it was before that. The previous owners had split it into four apartments and one sleeping room, so we figure it was hidden there for the tenant in the basement unit. 

Hey, I know! I'll see if it works in the basement door. We've never had to unlock it from the outside; we're always opening and closing it from inside. It won't mean a tinker's damn of course. But it will satisfy my curiosity.