What kind of key for this old door (mortise lock)? - Home Improvement Stack Exchange

I am helping someone with a lost key for her bedroom (interior) door; the door hardware is probably about 100 years old. There is clearly a lock and a keyhole -- see photos. What sort of key would

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Key stuck halfway in door lock - Home Improvement Stack Exchange

lock - Key stuck in Security Door - Home Improvement Stack Exchange

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What kind of lock set do I need for this door? - Home Improvement

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Can these locks be rekeyed to the same key? - Home Improvement

lubrication - How do I lubricate a sticky knob on an interior door? - Home Improvement Stack Exchange

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