19th Century Technology was designed to be infinitely accessible….. thus theoretically infinitely repairable.
The bad news: infinitely accessible means anyone with a pocket knife at hand or screwdriver in the kitchen drawer will try to adjust vintage door knobs with no concept of the proper original factory default settings.
This outlook leads to mangled screw heads, cross threaded set screw holes and damaged handle spindles.
Sometimes, with luck, it is possible to drill out and remove mangled set screws, reshape cross threaded set screw holes with the aid of a thread tap and restore the original factory default settings with slightly less accessible 21st Century technology so these door knobs may serve for another 50 to 100 years in quiet dignity.
Kattermann's Handyman Blog 