Very interesting information about the green seals, thanks! I just always assumed that different series had different shades of green.
Strangely enough, I happened to pick up a Series 1934 $50 Federal Reserve Note at the bank today (along with a nice Series 1950B $100 Federal Reserve Note), and the green seal is a different shade of green from the example that I already had in my collection. It's a lot more circulated, however, and now I'm wondering whether the ink ever changes color over time due to exposure to the elements?