I cannot take the "Z" for a sidewise "N", because of the style (the "N" is narrow and if you look at it sidewise, it will appear as a slightly squashed "Z"). And if I suppose that the "N" is the denomination letter, it belongs to the 5$ notes, not to the 20$. Strange enough that the printer letter does not appear...
If it's a legitimate error, it a very special one: wrong denomination letter, missing letter, misplaced serial, ink smear in the serial (both sides). I cannot exclude the possibility that it's a special case of mismatch prefix in which the denomination letter wheel has stepped two letters ahead (passsing from "L" to "N") before printing, and (why not) all the others wheels by 2 steps also. So the "original" prefix should have been *LB. If so, its a major misshaps by the quality control to let this error escape out. What are the position numbers? if they are under 55, this note is printed by CBN, so the complete prefix should have been ELB...
This error reminds me the mismatch serial number in the famous 10$ FDT****** , with the blackened squares in the serial...