I can't say for sure without looking at it up close but it's probably a [carbon mic](http://en.wikipedia.org/wiki/Carbon_microphone) or some kind of transducer.

edit, fixed link

[Auction link here](http://www.ebay.com/itm/RARE-Vintage-Martin-USA-R-18T-R-18-Tenor-Archtop-Acoustic-Guitar-/251015419984?pt=Guitar&amp;hash=item3a71af5850#ht_1328wt_1106)
[Backup imgur link](http://i.imgur.com/YvrL7.jpg)

My old man showed me this eBay auction with a gorgeous Martin R-18. It has these two, connected suction-cup looking things on the sound board. We're both really intrigued and would love to know what the hell is on this thing.

What kind of hardware is on the sound board of this 1930's Martin tenor?