Standard Shower/Tub Diverter Stem too Large

I've recently bought a house and in one of the bathrooms there's a hot/cold stem used in place of the diverter stem. So when you turn the knob to fill the tub water also comes out of the shower. I've gone to my local hardware stores; however, those diverter stems have a too large a diameter threading and will not fit into the body. I'm not sure if whoever did this job botched it and used the wrong components or if I just can't find the right diverter. Any help would be appreciated on trying to locate the correct diverter or direction on what I'd need to do to fix the botched job. I can provide additional information if required. Thanks

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