N30U is actually a 560 but a 500S is all I got!
One U-4B became a presidential transport aircraft for Dwight D. Eisenhower between 1956 and 1960. This was the smallest "Air Force One," and the first to wear the now-familiar blue-and-white livery.
Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.