There's quite a few Forum posts that have to do with identifying Birmingham Stove and Range CI. I came across a website today that has what I think is a pretty good guide. This may be old news for many members, but I haven't seen any references to this site. I see that they also have a Facebook page on cast iron cooking.
http://www.modemac.com/cgi-bin/wiki.pl/Birmingham_Stove_and_Range
http://www.modemac.com/cgi-bin/wiki.pl/Birmingham_Stove_and_Range