1925, 2c Carmine Rose, Postage
Due, "Zone Zone" Ovpt. (J19a). Position 18 from bottom right pane in
block of nine, natural s.e. at top, tropicalized original gum, bright
color, Very Fine and choice, scarce, of the 40 copies which were issued,
only approximately fifteen are known, the rest may be lost to philately,
with 1968 P.F. certificate as block of twelve and with 1970 A.P.S.
certificate as block of nine (Image)
1925, 10c Carmine Rose,
Postage Due, Vertical Pair, One Missing Ovpt. (J20a). Sheet margin at
top, top stamp missing overprint as always, small h.r., Very Fine and
choice, rare, only ten were printed, three of which are in the unique plate
block, with 1969 A.P.S. certificate (Image)
1925, 10c Carmine Rose,
Postage Due, "Canal" Wrong Font (J20 var). Position 51, bold grid
cancel, wide margins, Very Fine, scarce especially used
(Image) 
1925, 10c Rose Red, Postage
Due, Double Ovpt. (J20c). Lightly hinged, clear second overprint,
bright color, few natural gum skips, Very Fine, only one pane of 100 was
printed (Image) 
1930, 1c on 5c Blue, Postage
Due, "Postage Due" Omitted (J21a). Bottom plate no. 118117 vertical
pair, bottom stamp the error, h.r., deep rich color, small tear in sheet
margin onlyVERY FINE. THE UNIQUE PLATE NO. EXAMPLE OF THE "POSTAGE DUE" OMITTED VARIETY.
This variety was caused by a dramatic upward shift of the overprint. The "1" is split on all stamps, and "Postage Due" is omitted on the bottom five stamps of the pane.
Ex Plass. Illustrated in Canal Zone Stamps on p. 246. With 1967 P.F. certificate (Image)