1904, 8c on 50c Bister Brown
(15). Position 31 with broken second "A" of "Canal", slightly
tropicalized original gum described on accompanying certificate as part
original gum, fresh colorsVERY FINE. A RARE EXAMPLE OF CANAL ZONE SCOTT NO. 15.
It is uncertain whether 400 or 500 were printed. With 1975 P.F. certificate for block of four, this being the bottom left stamp. (Image)
1904, 8c on 50c Bister Brown,
"Panama" 15mm Long (15 var). Slightly tropicalized original gum
desribed on accompanying certificate as original gum, overprint shifted
downwards, rich colors, beautiful centeringEXTREMELY FINE. A SUPERB UNUSED EXAMPLE OF CANAL ZONE NO. 15 WITH ELONGATED "PANAMA".
Only 50 were printed. With 1983 P.F. certificate. (Image)
1904, 8c on 50c Bister Brown,
"Panama" Reading Down and Up (15a). Position 51 with part imprint at
left, left stamp with faint first "A" of "Panama", barely tropicalized
original gum, deep rich colors, almost perfectly centeredEXTREMELY FINE. A RARE EXAMPLE OF CANAL ZONE NO. 15 WITH "PANAMA" READING DOWN AND UP.
According to Canal Zone Stamps, this variety comes from Positions 1 and 51. Either eight or ten were printed. Eight are known.
Ex Plass. Illustrated in Canal Zone Stamps on p. 55. With 1967 P.F. certificate (Image)
1905, 8c on 50c Bister Brown,
"Panama" Reading Down and Up (18b). Top left stamp in block of four
with others normal, slightly tropicalized original gum, the variety with
faint first "A" of "Panama", rich colors, Very Fine and choice, only 350
were printed, Scott Retail as singles (Image) 
1906, 8c on 50c Bister Brown,
Both "Panama" Reading Up, "Panama" 15mm Long (19 var). Lightly hinged,
fresh and Very Fine, accompanied by normal stamp for comparison (Image) 
1906, 8c on 50c Bister Brown,
Both "Panama" Reading Up, "Zone" in Antique Type (19b). Position 48,
slightly tropicalized original gum, almost perectly centered, fresh color,
Extremely Fine, only an estimated 190 were issued
(Image)
1906, 8c on 50c Bister Brown,
"Panama" Reading Up and Down, "8 cts" Omitted (20c). Right pair in
block of four, Positions 55-56/65-66, numerous other varieties incl. bottom
right stamp misplaced entry of basic stamp, both left stamps 3mm spacing
between "8" and "cts" instead of normal 2-1/2mm, bottom left stamp with
antique "N" of "Canal" and also misplaced entry of basic stamp, barely
tropicalized original gum, small h.r. at bottom, rich colorsrich color,
choice centeringVERY FINE AND CHOICE. A RARE BLOCK CONTAINING TWO STAMPS AT RIGHT WITH "8 CTS" OMITTED.
The "8 cts" omitted error occurred on the right half of one pane of 100, thus 50 stamps were printed with the surcharge omitted. However, only ten pairs are possible containing one with and one without the surcharge.
Ex Plass. With 1970 A.P.S. certificate. Scott Retail with no premium for minor varieties. (Image)
1906, 8c on 50c Bister Brown,
"Panama" Reading Up and Down, "8 cts" Omitted (20c). Barely hinged,
sharp proof-like impression, almost perfectly centered, Extremely Fine Gem,
only 50 were printed with the surcharge omitted, ex Schay (Image)