I agree. I think this Syl nailed it, (pardon the architectural pun!) but it would really be great if this could be confirmed by an authority.
PPGS, would you consider emailing some photos to that compass museum posted earlier, or perhaps the MIT Museum of Engineering or some other technical museum? I think with such a rare and unique instrument, I think you will have no problem getting them to take a look at your photos. A museum specializing in architecture might also be a good option. *gives PPGS arm a twist*