The 1950-D Nickels were lower in number, so someone was trying to turn a Philly one into a 1950-D Nickel. It was was announced that the nickels would be lesser in number that year from Denver,
Philly: 9,796,000
San Francisco: 0
Denver: 2,630,030
So a lot of rolls got saved that year. Thus the prices in BU are just a bit higher than the circulated examples.
So there were no 'S' mints that year? (Another point I had noticed until I looked it up)