Oak Park has a simple plate with the mortise bearings that do a wonderful job. But, this can be done with any plate to be honest. I have to ask, are you sure it's the mortises that are off an not the tenons? Even an uneven edge or face could make them not flat.
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"Whether active duty, retired, national guard or reserve - is someone who, at one point in their life, wrote a blank check made payable to "The United States of America", for an amount of "up to and including my life". That is HONOR, and there are way too many people in this country who no longer understand it." -Author Unknown