but it was built as the house was errected around it. Not an after thought.
They had a fireplace, that shared the Den and MBR. So a fire could be warm and cozy for both rooms.
Installed in the middle of the fireplace that shared one chimney flume was a large steel trap door that was on double sided hinges that could be opened or closed on either side to block off one or the other. The blockade if you will was on a chain pulley device that would swing over and out of the way. This was hinged horizontally, but could much easier be hung vertically on one side.
They do have things that take care of just this occurance.
Ask a professional fireplace shop personnell, or call a sweep and see if he/she knows of the whereabouts.