70 Dart Thumbwheel Radio

Brought the radio over to a friend's house who also has a 340 Dart but unfortunately he doesn't have the factory radio in the dash anymore. But we figured out there is impressions in the radio that look like a paint cutter washer like the nuts for a seat track. The threads look to be 1/4-20 and the impression looks to be about a 5/8 O.D. washer. Can anyone confirm this? And where could I find these flanged nuts with the cutter washer?