Just a thought, but you could probably make replacement threaded serrated nut plates from some flat stock easier than findings originals. The above 1960-63 kits, which you can still find on ebay, are slightly different from the AD series serrated nut plates.
The 1960-63 serrated plates do not have threaded holes, but work with the smooth flat plates (below and right of the serrated plates) that have the threaded holes which sandwich the flat nut plate, serrated plate, cab panel and strike plate to lock in place when the hardware is tightened.
