How far should I drill and what size snap off screw should I use on a Vintage Hardwood Floor?

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Answers:
I would suggest 2" screws (length) and drill 1/8" holes. Snap screws are OK but I would recommend that you just use some self tapping course thread trim screws and just counter sink the small head. The trim screw will leave about the same size hole that needs filling. The trim screws have a head that requires a special hex head bit to drive them in. It's a shame you couldn't get access to the floor from the level below you. screwing up through the underlayment works so much better for squeaking floors.
I don't know about the glazing points in between the boards. Hardwood expands and contracts. There is always movement. Floor systems that make noise can come from many different reasons. I would even think that the underlayment used for a place as old as yours is most likely a T&G plank that also may be a contributing factor for your noise.