On the back of the one dollar bill there is an imprint of a fingerprint underneath the gap between the N and the E in one. On some notes part another print is visible next to it under the N. I see these prints on just about all $1 FRNs and some silver certificates. So the question is: who put their finger on the plates and is it a security device?
Uh....no. First, as funkee so nicely pointed out, it corresponds with Ole George's shoulder on the front. Second it is textured and generally has a small amount of BEP ink on it. Third I have seen this on notes that I have not handled directly. So for now I think I will stick with funkee's theory, but thank you for posting your thoughts!