"Supertone" is a brand name used by Sears Roebuck in the 1930s era and perhaps into the 40s.
Your mando might be similar in vintage to THIS ONE, although the one Elderly is offering appears to be a flat back. Tuning machines and other "hardware" may be similar enough to give you some clues about what to look for to fix yours.
If you Google supertone mandolin there are enough results to suggest that the brand still survives in a lot of attics, and several people appear to have restoration needs similar to yours. Since Sears was a seller, and not a builder, the Supertone name doesn't necessarily tell you who built the instrument, although they did have long term contracts with a few suppliers. Mandolins were very popular in the 30s, so there probably were lots of them sold (probably for as much as $25 new?).
The round back (tater-bug) body frequently presents a lot more time-consuming work to repair, simply because of the numerous joints that may need regluing, and you may need to look specifically for a luthier willing to work on one. Many good repair shops that will do a great job on a flat back may not have the tools (or somewhat specialized skills) needed to do a good re-assembly of a round back. Competent shops will tell you what they can do, and won't accept a job that's not within their capabilities; but it wouldn't hurt to be very specific about the back style when you inquire.