Although your find's not a slave tag, it's a neat item nonetheless. Given its lettering and the location where you found it, I'd say it's probably a claim check/tag of some sort from...
"Bathing Beach and Head House, City Point, South Boston, Mass."
Designed by Edmund M. Wheelwright in 1897, the popular bathhouse and restaurant reportedly derived its name from the fact that it was situated on the head of a peninsula in Boston Harbor.