1904, 5c Blue, Left "Panama"
2-1/4mm Below Bar (2 var). Position 36, slightly tropicalized original
gum, Very Fine and choice, this variety is found only on Positions 31 and
36, only 156 were printed from these two positions, and it is believed only
eight are extant, ex Plass, illustrated in Canal Zone Stamps on p.
18, signed J.C. Perry, with 1993 P.F. certificate
(Image)
1904, 5c Blue (2). Block
of four, unused (some tropicalized original gum), fresh and bright colors,
few short perfs at bottom right, otherwise Very Fine, scarce block, purple
backstamps, with 1973 A.P.S. certificate
(Image)
1904, 5c Blue (2). Left
margin block of four, Positions 21-22/31-32, bottom left stamp "Panama"
2-1/4mm below bar, traces of tropicalized original gum, bright colors,
light creases in top left sheet margin only, otherwise fresh and Fine, only
156 examples of this variety were printed, with 1968 P.F. certificate,
Scott Retail as singles (Image) 
1904, 5c Blue, "Canal Zone"
Inverted (2a). Large part original gum, brilliant color, Extremely
Fine, only 100 printed, some of which were used, the example offered here
is superb, faint Colman backstamp, with 1969 A.P.S. certificate (Image)
1904, 5c Blue, Pair, One
Without "Canal Zone" Ovpt. (2c). Left margin block of six with part
imprint, Positions 51-53/61-63, center stamp at top missing "Canal Zone",
lightly hinged, fresh and bright colorsVERY FINE AND CHOICE. ONE OF FOUR RECORDED EXAMPLES OF THIS VARIETY, AND THE ONLY UNUSED EXAMPLE AVAILABLE IN A VERTICAL PAIR WITH NORMAL STAMP.
Of the four recorded examples, one is used on cover, and two are the right stamps in a block of six. This is the only unused vertical pair known.
Ex Tows and Plass. With 1969 P.F. certificate. Scott Retail as pair and four singles of basic stamp. (Image)
1904, 10c Yellow (3).
Slightly tropicalized original gum, lightly hinged, brilliant color, Very
Fine and choice, with 1973 A.P.S. certificate
(Image)
1904, 10c Yellow, Left
"Panama" 2-1/4mm Below Bar (3 var). Tied by "La Boca * Canal Zone *
Jul. 12, 1904" handstamp on cover to Panama City, Ancon (Jul. 13 with "13"
inverted) and Panama Agency (Jul. 13) backstamps, rich color, Very Fine and
choice, only 100 were printed, of these only two unused and five used are
known, ex Plass, with 1969 A.P.S. certificate, Scott Retail as used (Image) 
1904, 10c Yellow, "Canal Zone"
Inverted (3a). Tropicalized original gum, brilliant color, choice
centering, fresh and Very Fine, only 200 printed, with 1976 A.P.S.
certificate (Image)
1904, 10c Yellow, "Canal Zone"
Double (3b). Top "Canal Zone" heavy impression, neat cancel, small thin
spot, faint corner crease at bottom leftVERY FINE APPEARANCE. ONLY TWO EXAMPLES OF THIS GREAT RARITY ARE KNOWN. BOTH ARE USED.
The other example of this variety has a lighter second overprint, and a faint cancel. In Canal Zone Stamps, the authors speculate that the second, clear impression of the overprint was added when the original overprint was deemed unreadable.
Ex Plass. Illustrated in Canal Zone Stamps on p. 19. With 1967 P.F. certificate (Image)