There is a point in the life of a mechanism where the ordinary answer stops applying. A part wears out, and the manufacturer no longer exists, or the pattern was discontinued thirty years ago, or the thing was never a catalogue item to begin with. At that point the choice is not repair or replace in the usual sense — it is adapt, substitute, or cut into a door that has never been cut into.
That call gets made a great deal in this town, and it deserves more care than it usually gets. A good number of the mid-century mortice mechanisms still working in Wellington doors are better engineered than most of what would replace them, and they can frequently be stripped, freed, re-pinned or have a worn component substituted and returned to service for years. Condemning one because the original maker vanished is lazy, and it costs the owner a great deal more than it needs to.
The opposite error matters just as much. Some mechanisms genuinely are finished — a body cracked, a case worn through, a keyway so far gone that a key will never be reliable in it again — and nursing those along is a disservice dressed up as thrift. Being straight about which category something falls into is most of what lock repair work here actually consists of.