According to a history of the Johnston family, Johnstonbaugh was one of the many names from the Scottish Clan of Johnston, just like Johnstone and Johnson. You might try the Johnston family forum (Johnstones and Johnsons list there, so I think it would be appropriate.) There are some really sharp genealogists there who might be able to steer you in the right direction. Good luck and happy hunting.
Doris Johnston James