There's no code breaker for those serials. All serials that are in the rear dropouts or under the bb shell that are like ADT0350 can only tell you the model name and some counting manufacturing number but nothing more.
Those serials were used until 1992. After that a new serial was used, which can be solved. It goes like this:
20X12930560 = 20", Adroit, made in Dec 1993, 560th frame made