It's also worth pointing out that for currency printed on LEPE, as this note is, the left serial number and the right serial number are applied by separate overprinting drums. The fancy new LEPE numbering heads are so large that they can't be packed close enough together to print both serial numbers on a note at the same instant. This means that the two serial numbers are receiving ink from different ink supplies, so if one of the two batches of green ink isn't perfectly mixed, you can get slight color variations from one side to the other. As Steve said, it's not an error, at least not when the difference is this subtle--but it is
a new effect, that wasn't generally seen on COPE-overprinted notes.
After reading this, I referenced it with an email exchanged between myself and Derek Moffitt. My original question to Derek was, “Do you know what series the BEP started engraving the Fed Seal instead of the Fed Seal being applied during the overprint (3rd print).”
This was his reply,
They didn't. Engraved Fed seals were used through Series 1934D; overprinted Fed seals have been used for everything from Series 1950 onward, *except* the 1996-generation $100's. I'm not absolutely sure, but I think the reason has something to do with not having enough three-color overprinting presses yet in 1995-6 when production of those $100's began.
(FRNs of the small-head designs have a two-color overprint, green and black. More recent FRNs have a three-color overprint where two of the colors are green; the Treasury seal is inked separately from the serial numbers, and wouldn't have to be the same color. The 1996-generation $100 has a two-color overprint where both colors are green; there's no black overprint at all, since the Fed seal is engraved in the intaglio plates and the district number is green instead of black. By the way, currency overprinted on LEPE now actually has a *four*-color overprint where *three* of the colors are green: the left and right serial numbers are printed separately, because the new computerized numbering blocks are so large that they can't be crammed together close enough to print all the serial numbers on a sheet in one pass.)