In 1873, the contract for the printing of the U.S. postage stamps was awarded to the Continental Bank Note Company. At that time, Continental took over some of the dies and plates used by the National Bank Note Company who had held the previous contract. In order to establish whether certain stamps had been printed by National or Continental, "secret" marks were added to many, if not all of the plates used to produce the Continental banknotes.       It must be noted that a "secret" mark has never been found on either the 24¢ or 90¢ stamp, and only on the American Bank Note 30¢ stamp. In fact, the 24¢ Continental is impossible to distinguish from the 24¢ National, other than a possible few copies printed on ribbed paper, presumed to have been used only by Continental. Only one stamp has ever been certified as the Continental, and it is of course clearly on ribbed paper. On many of the higher denominations the quality of the printing plate is evident; often the National stamps show a finer detail, especially on the earlier printings while the plates were still new. Many of the stamps are distinguishable by the color of ink used, and in fact, the only way the thirty and ninety cent stamps can be distinguished is by color.       The best way to recognize the "secret marks" is to actually see them. illustrations are shown below

	1c Blue - Franklin
	There is absolutely no blue in the ball left of the numeral "1". Even the slightest hint of a blue dash will relegate this stamp to Continental or American status, since the secret mark often is very light.
	If there is even the slightest hint of blue in the ball left of the numeral "1", this stamp must be considered a Continental or American Bank Note printing.
	2c Brown - Jackson
	There is clearly NO colour joining the line and ball to the left of "U.S." Note that the types can also be distinguished by the color. The Nationals are nearly always a reddish brown. This is a difficult issue to differentiate and a reddish brown color without the "gap" should be considered a Continental.
	There is a very light amount of color joining the line and ball to the left of "U.S." Since it is widely held that the "secret mark" does not appear on all Continental printings, this stamp must be distinguished by the color. The Continentals are nearly always a brown or dark brown. The printings in orange should not present a problem, and in fact may provide an excellent way to get a feel for the secret mark, since they MUST have the mark
	3c Green - Washington
	There is clearly only light shading below the upper ribbon, below "THREE". Note that there is no heavy shading at the point where the two ribbons join.
	There is shading below the upper ribbon, below "THREE". Note that there is heavy shading at the point where the two ribbons join
	6c Dull pink - Lincoln
	There is no extra shading in the lower ribbon, below "SIX". Note that the colour is a much richer, brighter shade of carmine than the later Continental printings
	Four lines of shading have been reinforced in the lower ribbon, below "SIX". Note that the colour is a much duller shade of carmine than the National printings and the impression is generally weaker
	7c Orange - Stanton
	There is no "C" in the top corner of the lower right ball
	There is a shaded "C" in the top corner of the lower right ball. This is one of the easier "secret marks" to identify
	10c Brown - Jefferson
	There is no semi-circle in the ball at the end of the right hand scroll
	There is a semi-circle in the ball at the end of the right hand scroll below "POSTAGE"
	12c Violet - Clay
	The circular balls in the numeral "2" are round
	The circular balls in the numeral "2" are crescent-shaped
	15c Orange - Webster There is a lot of debate over the secret mark on this value The following is considered the best method for seperation
	There is a distinct definition in the fine lines to the left and right of the numeral "15" and also the definition of the lines in the upper triangles. According to the noted colour study of R. H. White, this stamp is usually a brighter and often darker orange than the Continental or American printings and is listed in his colour encyclopedia with both orange and bright orange variations. Looking at White's illustrations under a full-spectrum light, it has been noted that some of the Continental and American illustrations have a distinctly reddish tint, while the National printings do not
	There is poor definition in the fine lines to the left and right of the numeral "15". This stamp is usually a duller orange, and is found in the range from yellow to red orange. White lists these stamps as having the following color variations: yellowish orange, pale orange, reddish orange (3 shades) and orange. The American stamps, in addition to being on the porous paper, often tend toward the reddish orange. It is thought by some that the only definitive way to separate a Continental from a National printing is if the stamp is on ribbed paper. This same distinction is also made for the rare 24¢ stamp.
	30c Black - Hamilton
	The upper triangular protrusion in the letter "S" of "CENTS" is narrow and distinct. The National printings are a full, rich black and the Continental printings are a duller to even greenish black
	The upper triangular protrusion in the letter "S" of "CENTS" is widened and less distinct. This "secret mark" appears on some but not all of this issue
	90c Rose-carmine - Oliver Perry No "secret mark" is known on this issue The best method known for seperation is detailed below
	National Bank Note Company The color is richer and has a slightly bluish tinge. Most texts refer to this color as carmine or dark carmine
	Continental Bank Note Company The color is less rich and has a slightly yellowish tinge. Most texts refer to these colors as rose, rose carmine or pale rose carmine
	American Bank Note Company The color may be similar to either the National or the Continental Printings. In this case the rich carmine color resembles that of the National printing