Toilets, Sinks, Showers, Tubs and Disposals - Old tub drain replacement

I have an old tub (1950's I think) with a drain that I would like to replace. The manufacturer was American Standard I think and currently there is no grate or covering to prevent hair and other objects from going down the drain. I'm not sure how to remove the old drain. I have been to home depot to look for replacemnts, but I didn't see any designs that are the same as what I have. It doesn't appear that there any screws holding it in, but there are two square metal notches sticking out just a little on the sides of the metal drain as it goes down into the tub. It might be threaded, but I can't tell and am wondring if these two small square notches fits some tool that allows the drain to be turned. I can E-mail pictures if that helps.

Any advice would be appreciated!