I found this tag the other day. It's brass and almost exactly the size of a quarter. The picture shows what is on the front (B.& O.R.R. 6598) and the back (M. Of W. B).
I tried to do a search on it but didn't find much.
Nice find.
You may also want to consider contacting the... B&O Railroad Museum
Baltimore & Ohio Railroad Museum
901 West Pratt Street
Baltimore, MD 21223
Phone: 410-752-2490
Email:info@borail.org
Date wise, I'm thinking 1900 - 1920.
Dave
Do not know the year but those hung on the coupler on the rail car! Cool find, worked in a steel mill in columbus ohio made all the parts under the rail car!