If you do go mirror-lid, you will need to reverse the leads to the yoke's horizontal windings. Otherwise, the picture will appear backwards reflected by the mirror. If you look directly at the screen of a working TRK-12 while standing in front of the cabinet, the picture appears upside-down. The vertical inversion is corrected by reflection from the mirror, and the horizontal reversal is corrected by the polarity of the yoke leads. Oh, those clever RCA engineers of the thirties!