.disc files can be Toast project files, with only a description and links to the content, as you can probably tell from the file size.
Without the original linked files, the project files are useless. The project file would not be something to archive.
You would normally open the .disc files from the File﹥Open menu, or by double-clicking the file icon. That part of Toast hasn’t changed at all.
It could very well give difficulties across versions, as small changes in how the application wants information for a project, might render the old format inaccessible.
In early 2007, Toast version 8 was released. Is Toast 7 or 8 a likely candidate for creating that file back then?