I am by no means a cast iron expert, but I like mysteries. So, I have been doing a little research about your question. As @Max points out, a picture would be very helpful, but a raised straight mark on the bottom of your pan is known as a "gate mark". Gate mark casting was used in the 1800s, and could very well signify that you have quite an old piece. The line comes from the piece actually "breaking the mold". The ring, if it is also raised, is known as a heat ring. Since these pans were made to work with wood fired stoves, the ring was meant to raise the piece slightly above the surface.
I have been unable to locate a reference to "BR". Is it possible that the B is actually an 8? Numbers were often cast into vintage pieces to indicate size. Some companies added letters, known as "pattern letters", to indicate the mold pattern. Could it be 8 R?
There is lots of good information here.
From your brief description, it appears possible you might have a piece that is quite old, perhaps from the early 1800's.
Given your photos in the update, it's tough for me to tell if it is an 8 or a B. I've been unable to locate any images that match your pan. However, using the link above you can work your way through a list of foundrys. Many do not have pictures, unfortunately. I've scanned the most obvious ones, thinking that if it is a "B" it might be related to the name of the producer. No luck.
However, that site has a link to a discussion forum. Perhaps you could post your pics there. That way, a real expert could point you in the right direction.
By the way, if you do identify it, please return to add your answer here.