Yep, Joe is right, all the information you're going to get is probably right there in front of you already. There were probably hundreds of small gunmaking firms operating in the area at the time, with little to no records remaining. You might be able to contact the City Hall in the town shown on the barrel & ask them if they have any records dating back that far. Certainly looks like a light hunting/stalking rifle, not uncommon for Germanic hunting rifles to have set triggers & aperture sights. There are two towns in what is now Germany (remember, Germany wasn't unified as a nation until 1871, almost certainly after this rifle was made) with the name Zella. Zella-Mehlis, & Zella-Rohn, that appear to be in neighboring states, so it could probably be from either. Nice looking rifle, caliber would probably considered to be 10mm if your measurements are correct. By the way, does the aperture sight base have a round or square hole in it?