This knob is likely same vintage as the house - 1956. I've installed many newer knobs over the years, and know about what you're talking about for sizing, but that's not what this is. Like I mentioned, there's a 'bit' that goes into that square hole, the bit has a flat head style top, and when the knob is properly installed, the flat head is available to accessed - leading me to believe it's what will unlock the door. Normally.
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