1906, 2c Red & Black,
Horizontal Pair, Imperforate Between (23a). Top margin block of four
with imprint, barely tropicalized original gum, lightly hinged, intense
colors, beautiful centeringEXTREMELY FINE. A SUPERB TOP MARGIN POSITION BLOCK OF THIS PERFORATION ERROR. ONLY 20 ERROR PAIRS WERE PRINTED IN TOTAL.
According to Canal Zone Stamps, two panes of 100 were discovered with the center vertical row of perforations omitted. Of the 20 pairs printed of this variety, only two position blocks such as this are possible.
Scott Retail as two pairs. (Image)
1906, 2c Red & Black, Ovpt.
Reading Down, Vertical Pair, One Without Ovpt. (23b). Top stamp missing
overprint, slightly tropicalized original gum, h.r., few sensibly
reinforced perf separations, intense colors, Very Fine and choice, only ten
pairs were printed, the error was caused when the top row of the sheet
missed receiving the overprint, with 1969 A.P.S. certificate (Image) 
1906, 2c Red & Black, Ovpt.
Reading Down, Vertical Pair, One Without Ovpt. (23b). Top stamp missing
overprint, tropicalized original gum, h.r., few perf separations sensibly
reinforced, bright color, Very Fine, rare, only ten pairs were printed,
purple Colman backstamps (Image) 
1906, 2c Red & Black, Ovpt.
Reading Down, Double Ovpt. (23c). Bottom margin with part
imprint, slightly tropicalized original gum, h.r., deep rich colors,
clear second overprint, Very Fine and choice, scarce position single, only
one pane of 100 was printed (Image) 
1906, 2c Red & Black, Ovpt.
Reading Down, Double Ovpt, One Diagonal (23d). Tropicalized original
gum, intense colors, light gum soaks, otherwise Very Fine, scarce, only
part of one pane is known to have been printed with this variety (Image) 
1906, 2c Red & Black, Ovpt.
Reading Down, "Canal" Double (23i). In pair with normal, tropicalized
original gum, intense colorVERY FINE AND CHOICE. ONLY THREE EXAMPLES ARE KNOWN.
Of the three recorded exampoles, only this one and one other are in a pair with normal stamp. Ex Plass. Illustrated in Canal Zone Stamps on p. 77 (detail). With 1970 A.P.S. certificate. (Image)
1906, 5c Ultramarine & Black,
Double Ovpt. (24c). 6-3/4mm spacing, tropicalized original gum, deep
rich colors, fresh and Fine, only 200 were printed (one pane with 6-3/4mm
spacing, one with 7-1/4mm spacing) (Image) 
1906, 5c Ultramarine & Black,
"Canal" Only (24d). Part tropicalized original gum, h.r., fresh and
bright color, light corner crease at bottom rightVERY FINE APPEARING EXAMPLE OF THIS "CANAL" ONLY VARIETY. OF THE TEN THAT WERE PRINTED, ONLY THREE ARE KNOWN. A GREAT RARITY OF THE ISSUE.
According to Canal Zone Stamps, one pane of 100 was printed with the overprint shifted left, so that the right vertical row received "Canal" only and the other 90 stamps received the "Zone Canal" variety, Scott 24e (two of the 90 are known). Only three copies of Scott 24d are recorded: 1) single with part sheet margin at right, ex Plass lot 486, 2) in rejoined horizontal pair with "Zone Canal" variety, ex Plass lot 487, and 3) the example offered here. (Image)