1918, 2c Vermilion & Black
(53). Left margin with part of "Zone" printed in sheet margin,
barely disturbed original gum, rich color, Very Fine and choice, only 20 of
this variety exist and come from the sheet which caused the horizontal
pair, one missing overprint variety (Scott 53b, see next lot), Scott Retail
as normal (Image) 
1918, 2c Vermilion & Black,
Horizontal Pair, Right Stamp Without Ovpt. (53b). Horizontal strip of
four, right stamp missing overprint, large part tropicalized original gum
brilliant color, beautiful centeringVERY FINE AND VERY RARE. ONLY 22 PAIRS ARE KNOWN TO EXIST.
The error was caused when the sheet was misfed into the printing press, and the rightmost vertical row did not receive the overprint. Two additional copies are known caused by a foldover. An example of the overprint printed in the sheet margin from the same sheet is offered in the previous lot. (Image)
1920, 5c Deep Blue & Black
(54). Mint N.H., sharp proof-like impressions, perfectly centered,
Extremely Fine Gem, Scott Retail as hinged
(Image) 
1921, 1c Light Green & Black,
"Zone" Only (55d). Left margin horizontal pair with normal, "Canal"
clearly visible in sheet margin, lightly hinged, left stamp with vertical
foldover creases which caused the error, rich colorsVERY FINE AND CHOICE EXAMPLE OF THE "ZONE" ONLY ERROR. ONLY FOUR ARE KNOWN, ONE OF WHICH IS USED.
This error was created by a foldover in the left vertical row of one pane, which caused "Canal" to print in the sheet margin. The following examples are known: 1) the example offered here, 2) unused in horizontal pair with normal, part of "Canal" visible at left, ex Plass lot 581, 3) single contained in a large unused block, and 4) left stamp in horizontal strip of five used on cover, ex Plass lot 582. (Image)
1920, 2c Orange Vermilion &
Black, Double Ovpt. (56a). Clear strike of second overprint, almost
perfectly centered, Extremely Fine, only 100 were printed (Image) 
1920, 2c Orange Vermilion &
Black, Horizontal pair, Left Stamp Missing Overprint (56d). Left margin
horizontal strip of five, left stamp missing overprint, h.r., brilliant
color, fresh and Extremely Fine, only 20 were printed
(Image) 
1920, 2c Orange Vermilion &
Black, "Zone" Double (56f). Sheet margin at left with part of "Canal",
intense shade, barely hinged, Very Fine and choice, scarce, of the 30
printed only ten show part of "Canal" in selvage
(Image) 
1921, 5c Deep Blue & Black
(57). Slightly tropicalized original gum, rich colors, beautiful
centering, Extremely Fine, scarce in such superb condition (Image) 
1921, 5c Deep Blue & Black,
Horizontal Pair, Right Stamp Without Ovpt. (57a). Bottom right corner
margin with narrow selvage at right and huge selvage at bottom, barely
tropicalized original gum, h.r. reinforces a few internal perf
separationsVERY FINE AND CHOICE. THE UNIQUE BOTTOM RIGHT CORNER MARGIN PAIR CONTAINING ONE STAMP WITHOUT OVERPRINT. ONLY TEN PAIRS WERE PRINTED WITH THE RIGHT STAMP MISSING THE OVERPRINT.
According to Canal Zone Stamps, the missing overprint occurs on the left pane of the sheet of 200, as evidenced by the narrow selvage at right and the existence at one time of a horizontal strip of ten with wide selvage and "Canal Zone" printed at left.
With 1978 A.P.S. certificate (Image)