Identify mark on cast iron skillet
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 &quot;gate mark&quot;. 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 &quot;breaking the mold&quot;.  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 &quot;BR&quot;.  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 &quot;pattern letters&quot;, 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 &quot;B&quot; 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.