Old-style tub: drain stopper help needed

I have just moved into an old house built circa 1925 in Brooklyn. The bathtub does not have a tab near the top of the tub to stop water flow for a filled bath. There is a grating with verdigris over the drain which is screwed tight.
Right next to the tub is a metal tube inserted into the bathroom floor, inside that tube is a metal rod that I can lift and jiggle around. What is it and what does it do? It doesn't seem to stop water flow if I should happen to want a bath. The rod keeps falling back in place when I pull it up. I cannot pull it out completely.
Anything to be done short of removing the grating and inserting a rubber plug? I'm sure that metal rod and tube are for SOMETHING.
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Amy Chan wrote:

It was called a "Tower Drain" waste and overflow. http://www.vintagetub.com/asp/product_detail.asp?item_no=s48p
http://www.shop4classics.com/product.asp?lid 27
http://www.ohbath.com/Products/index.cfm/Product_Group_ID/1/Product_Category_ID/25/Start/11
for a few pics.
The "stopper" is a brass tube within a (outer) tube. The inner tube usualy has some sort of rounded or decorative "knob" on top. When lifted up and turned slightly, it will lock in position and allow water to drain out. When turned so that the tube drops down, the bottom end of the inner tube mates with a seat inside the TEE fitting and stops water from draining.
I can't tell from your description whether some part is missing or not. Perhaps the pics will help you decide. Perhaps contact some neighbors who have a similar installation. If the parts are there, the seating surface on the tube may need cleaning.
Jim
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