RED & BLUE Seals question

[Dozer D]

I'm not much into currency notes, just every now & then if something strikes me unusual. But need to know what is so special about RED & BLUE seals on bills? One of my coin tellers gave me a few Red Seal $2-bills. Yes they look different in appearance, but what else is there?

 

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[cjweber]

They are an older series of federal reserve notes... blue seals on note are silver certificates...