bathtub drain question

I've just bought a 15 year old condo apt. It was rented out prior and was in fairly tough shape. I've been working on replacing a broken wall tile in the tub and just noticed that the strainer is missing on the drain.
The tub seems to drain alright but I'm wondering why that strainer would go missing. From what I've been told those strainers just spin out, but I'm thinking that they may hold the drain up to the base of the tub.
Any help/advice appreciated.
Peter H
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Peter H wrote:

There's a hundred different designs. But, you're right, the drain fitting on top screws in and holds the bottom "shoe" against the tub (with a rubber washer).
See a variety here: http://www.briggsplumbing.com/products/faucets.asp?cid cessory&scid If possible, avoid unscrewing it; you may not be able to correct leaks later.
Jim
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