help with tub drain assembly

[Dixie722]

I had to destroy the drain in my 75 tradewind to get the shower pan out, there was just no way to get the ring out of it. I need to know what size drain assembly to get, my sewer pipe is intact, and still has the female threads. So I am looking for the male part, that you see when in the tub. thanks for any help.. it is holding up the re installation. One more question.. what is the size of the retaining nut on the back side of the drain ?