Am. Std. and Price Pfister are 2 of several companies that used this design at one time. There are acouple of different threads that were used and adapters can be located but they usually end up loose and leaking.
I would recommend unscrewing the shower arm out of the drop eared elbow in the wall and replacing the shower arm and showerhead with the version used today. Note the shower arm may shear off inside the wall when you attempt to do this. If that happens its not the end of the world just post back and we'll tell you what to do.