The Red S:
To make printing more economical, there were several types of currency that were experimental issues. Of these, the latest we are aware of is the famous Web Press notes that were printed on a different type of printing press. These were last printed in series 1988A, 1993 and 1995. But there are several other types out there as well. Some are denoted only by the series year and the alpha characters on either side of the banknote, such as the 1928B YB and ZB banknotes. But in WWII (and in this notes case), there was a red R and a red S printed on the lower right hand side of the front of the notes. 1,184,000 of each type R & S were printed and released into circulation. Evidently, there was no useable data to make the experiment worthwhile, but the nice R and S does make them sought after notes.
I've tried digging up more information on the different details on the experiments, but have only came up with the 1928B when they switched to 75% linen and 25% cotton recipe.