Pipe threads: Help identifying?

I replaced my bathtub drain , but it leaks . It doesn t leak when showering, only when draining a full tub. I gathered it was leaking through the threads when the water is backed up and draining slowly . After removing the drain I realized the threads are non continuous . On two opposing sides of the diameter... show more I replaced my bathtub drain , but it leaks . It doesn t leak when showering, only when draining a full tub. I gathered it was leaking through the threads when the water is backed up and draining slowly .

After removing the drain I realized the threads are non continuous . On two opposing sides of the diameter there are flat spots that run the length of the fitting perpendicular to the threads, effective cutting the threads off completely in those areas. I think the water is leaking up this path.

The heart of my question : was it designed this way intentionally ? Were they assuming it would only be for showering? Or is this a machining defect? If it s not a defect , what is this type of threads called ? I ve scoured the internet seeking the answer to no avail !
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