You are bidding on a PMG graded 1985 $1 Offset Print error note, graded a desirable CU-64 EPQ.  Many offset prints may only have partial printing, this one however, is richly covered in green, giving the collector a dramatic example of this error.

Offset printing notes happen when there is no paper between the printing plates, and the plate imparts its ink onto the other plate.  Subsequent printing will have that ink show up mirrored on the opposite side of the note.  

The most common offset print errors are of the Front on Back variety, and not the Back on Front variety such as this one.  This is due to quality control.  Quality control often looks at just the front of the notes for errors, so the likely hood of this kind of note to be caught is very high.  This one however, made it past quality control, and is now available in high grade.