Even in the first year she printed 500 copies a month of her letters ("little bluebacks" the sailors called them, on account of the colour of their cover); but before many years had passed as many as 21,000 a month were printed and circulated.

Then the sailor boys wanted a letter all to themselves, saying they could not fully understand the men's bluebacks.

[Science of coins] numismatics, chrysology^. [coin scholar or collector] numismatist. paper money, greenback; major denomination, minor denomination; money order, postal money order, Post Office order; bank note; bond; bill, bill of exchange; order, warrant, coupon, debenture, exchequer bill, assignat^; blueback

Yes ma'am I remember's McGuffey's and the Blueback speller too.