On old safes it is common to run across odd bolts/screws. While some thread standards were being developed in the late 1800's, SAE standards were first set around 1915. The Unified Thread Standard and the ISO metric standard were not implemented till the 1940's. On 1800's safes you can run into almost anything. When you consider the times, the safe factories were manned with immigrants and even the safe designers had ethnic backgrounds. Fasteners were made to the design they were used to. With minimal standardization in place it was easy for someone to come up with a "better" bolt.
On the old Moslers it is often a common TPI (threads per inch) but it is applied to non standard diameter. It can also sometimes be a British Whitworth thread that uses a 55° profile vs the UTS 60° profile but they will interchange. Except for the 1/2" size which uses a different TPI. There can be some slight interference that can be overcome. They shouldn't be metric.
With the TPI's on an odd diameter you will be forced to drill and tap to a standard thread or have a new bolt made to fit. In non critical areas an update may work, if cosmetics or application dictate an original then a new bolt will have to be made on a lathe.
I recently had to make a new bolt for a 1870's Martin C. Briggs safe. It not only had an odd combination of TPI/diameter, it also had an odd height, crown, and slot to the head. I had to advance my basic machining skills up one more notch.
