I worked for a company that installed acrylic tub liners in people's homes, and we had approximately 30-40 different tub drains on board the truck. They were different diameters, different lengths, different threads (coarse, fine, etc.). There was no way to tell which one was which until we took the old one out, and compared it to the ones on the truck. If you can get it out, you should take it to your local plumbing supply store (might need to be a specialty store), and ask them to match it for you.
