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Hi again! It is probably a glass counter from the late 1800s. Workers at glass factories used to use glass rings as counters to keep track of their output.
Here is a link to more of these glass items you have there. An antique glass insulator.
In the image I linked to, the smaller round item attached at the top is the knob (Small cylindrical insulator used to secure house wiring or the cylindrical member projecting out from the insulator body for tying the line wire.), missing from yours.