Very Fine - Julian/Morgenthau FR-1610 - Solid VF with good paper quality. Seldom encountered "S" Experimental Blue Seal US Silver Certificate. It has been said in the case of experimental notes a red R was overprinted on the regular paper and a S was overprinted on the special paper. But recent information has come up that the "R" really stands for Rayon and these are the experimental notes, while "S" means Silk. During WW 2 a large amount of silk was needed to make parachutes. So it would be likely that this was a wartime experiment to replace the silk threads used in US currency. The notes both R & S were delivered June 20th 1944 and issued into circulation to test there comparable durability. No conclusive results were determined from the experimental issue. Still a neat part of history!