Found old key, please help identify it?

[Marsquar]

I found this old key today while mowing the lawn, in a stony area. I believe it was previously buried and recently came to the surface. It is 7cm long, solid and weighty. I tried looking online and found two very similar pictures, one identified as a prison key and the other identified as a safety deposit box key. I am interested in knowing if either of those could be correct, or if it is something else altogether. There are no markings other than the obvious number at the top and the same number in mirror image on the bottom edge (which is somewhat damaged). The mirror image is particularly fascinating to me. I don't know what kind of metal it is.