Very Nice Find!!!!

I've been detecting in the low country for about 3 years now but have never found a slave tag. Yours was obviously on private property as I've been told by a City Offical that all parks in the Charleston area where most of the badges were used are closed to Metal Detecting.
Sounds like you've done your homework and you may already know that one of the currators of the Charleston Library wrote a book on Slave Tags about 3 years ago in collaboration with a local Dentist. I'm sure its still available.
You've got my vote for best find!