UGotIt, the significance of the note is that Joseph Barr served as Secretary of the Treasury for a VERY short period of time. His signature never made it to any of the higher notes so you can only find it on the $1 bill and therefore there isn't an abundance of them. I believe they currently go for 4 to 5 times face value for uncirculated (feel free to correct me here guys). I also think it was only on $1 notes with the branch-letters B, E, G, J, and L. Finding one in circulation is a sweet find indeed.
More Info:
Joseph W. Barr was the 59th Secretary of the Treasury and served under President Lyndon B. Johnson from Dec. 12, 1968 to Jan. 20, 1969. This was the last 28 days of the Johnson Administration.
Hope that helps
-TwoYewts