14 ga double gun so 0,74" SN 17688
with " a single leaf folding rear sight" probably Rotationsstück in one or two barrels

http://jamesdjulia.com/item/386-122/
waiting for the official Hermann Historica online catalog, the flyer http://www.hermann-historica.de/auktion72/pdf/flyer/A72_flyer.pdf
showing interesting stuff so a pair of Franz v. Dreyse duelling pistols in the box similar to another needle fire pistol sold in italy 2010

Goergen's Gun Shop was so kind to send me more pics for the combination gun SN 15247
http://www.goergensgunshop.com/index.php?p=1_9

to bad its not readable anymore
I believe this gun was changed later by Dreyse, its having centerfire extractor's that did not start before SN 23xxx and the holes in the action smelling like firing pins instead of needles.