Four varieties are known:
1. Red back
    Fine $15.00 | VF $20.00 | EF $30.00 | AU $40.00 | New $70.00 | Gem New $140.00
2. Red back with a small letter "a" on the front
    Fine $20.00 | VF $30.00 | EF $40.00 | AU $50.00 | New $100.00 | Gem New $170.00
3. Green back
    Fine $15.00 | VF $20.00 | EF $25.00 | AU $30.00 | New $40.00 | Gem New $75.00
4. Green back with a small letter "a" on the front
    Fine $15.00 | VF $20.00 | EF $25.00 | AU $30.00 | New $50.00 | Gem New $90.00

Spencer M. Clark (1810-1890) served under Abraham Lincoln as the the first Superintendent of the National Currency Bureau (now known as the Bureau of Engraving and Printing).  He had his own portrait placed on these notes, an act that (once discovered by his superiors) led to legislation preventing a portrait of a living person from appearing on Federal currency.  A small letter "a" appears on the left side of Varieties 2 and 4, but the size and placement of the letter may vary.  These notes were printed in sheets of 20 (4 up by 5 across) and the "a" appears only on the notes from the farthest left column.  Theoretically, they should be five times as rare as the notes without the letter "a".

Specimen Printings are known:
1. Front of the above varieties
      Narrow Margins $65.00 | Wide Margins $170.00
1. Back of Variety 1
      Narrow Margins $50.00 | Wide Margins $175.00
2. Back of Variety 2
      Narrow Margins $50.00 | Wide Margins $150.00

Some of the Specimen notes were printed on blank paper captured from blockade runners during the Civil War.  Prepared for the Confederate States of America for use on their currency, this paper features a distinctive "CSA" watermark.  Parts of the watermark are often visible on the Specimen notes.