So every resource online says “A bathtub drain is always 1-1/2”. And 1-1/2" bathtub drains are all that Home Depot seems to sell. Problem is my 1960s bathtub appears to be 1-3/4" despite all the resources say bathtubs are never, ever, not in a million years something other than 1-1/2". My “solution” was to put a 1-1/2" drain in, the only one I can buy, and try to plug the massive resultant leaks with plumbers putty. But every six months this gives out and I get water all over the basement floor.
I’m an idiot DIYer. Redid my 1915 bathroom. Nothing from Home Depot fit. The local plumbing supply store was quite helpful. They didn’t have a 1915 toilet ring but they did have an adapter. Same story with the claw foot tub.
There were many many many different thread pitches for drains in the past. I have a display with about a dozen different adapters at work. In 7 years I’ve sold One successfully to keep the existing drain.
A good plumbing supply house will have the adapters but I can tell you it’s so much easier and cheaper to just pull the old shoe and replace it with a standard 1.5” one. Even if you need to open a wall or ceiling and patch, you will end up with a much better draining tub and a waste that is easily and simply repaired in the future.