ALL "slave tags" were made of thin copperbrass sheetmetal -- so your iron disc absolutely cannot be a "slave tag."
The very-few actual genuine "slave tags" which exist were manufactured in Charleston South Carolina, for use in that city's "slave hire system," and have that city's name stamped into the copperbrass sheetmetal tag.
Unfortunately, fake ones are all over the place, particularly seen for sale on Ebay. See the photo below, which shows a few of the many varieties of fake ones.
Remember, all of the genuine ones were made of thin copperbrass sheetmetal, and all say "Charleston" (with some other info such as the slave's job of Carpenter, or Porter, etc) on the tag.
http://www.treasurenet.com/forums/general-discussion/238093-regards-slave-tag.html
Additional educational info about genuine "slave tags" is in the following discussion here at TreasureNet:
http://www.treasurenet.com/forums/what/225162-could-possibly-slave-tag.html#msg2749557
You asked about how to clean the rust-encrustation off of your iron disc. I know TreasureNet contains info on cleaning iron relics, but I do not know where. Perhaps somebody else here can provide a link to that info at TreasureNet. The two iron-cleaning methods I use are Electrolysis and Zinc-&-Lye Bath.
