P15 body number tag

[BobT-47P15]

All these body number tags that I've seen are painted body color and have a small piece of wire fastened into the hole on lower left end.

When my son in law repainted this car after the tornado (which was 10 years ago this year), he did the tag this way.   At first I didn't like it

but I've decided it's not so bad after all......does stand out and is more readable.    But I still don't know what the little piece of wire means.  

Anyone know?