Judging by the application and era (old MS-DOS) *.tbc is probably a fixed length binary record format written by the application's developers.
Try opening up the file in a text editor like TextPad first and if you can read the contents, if so I have a fixed length text file reader that you can adapt to your needs. If you cannot you may need to determine field lengths and data types through trial and error.
Also, are there associated files for each *.tbc? A paired file could indicate field lengths and data types (or that information could be stored at the top of a *.tbc file itself).