Hahn and Landeron evolved to become one company. Despite innovating the cam-based chronograph design, they also owned the original column wheel chronograph patents through their relationship with Heuer. Single button 15 1/2 ligne Hahn Chronographs, typically from the 1920’s and 1930’s, can be especially beautiful and enjoyable watches but require a good deal of attention and care in servicing and restoring. First, for Calibre references, there were different numbers used that do not necessarily align with what you see online or in books. For parts compatibility between models, compatibility exists in some cases but not in others. The original balance jewels used in these watches were thinner than today’s modern ones. As a result, for some models, if changing the balance jewel, the staff pivot cone can bottom-out on the new jewel prior to the pivot reaching the cap jewel, creating a frustrating path to resolution. Not unique to these watches but sensitive in them is the angle of the pallet bridge jewel setting— they were not always installed flat with the bridge. These watches used powerful mainsprings and saw lots of use. Typically I need to change most of the jewels and bushings on these watches due to the hard driving mainspring and wheel force. When fully serviced they can run well and be very robust and are quite beautiful.