Back printing with an associated plate number on a United States federal duck stamp began with the $1 Redhead Ducks stamp of 1946, Scott RW13. The purpose of the back printing was to inform hunters of the requirement to sign their stamps in ink on the face, ostensibly to prevent reuse by another hunter. The text, printed on the back of each stamp, reads: "It is unlawful to hunt waterfowl unless you sign your name in ink on the face of this stamp." The same plate, No. 47510, was used for the back printing on Scott RW13-20. It was necessary to apply back printing and the accompanying plate number using an offset press. The text and number were printed directly on the stamp paper, which was then gummed. Beginning with the $2 Ring-necked Ducks stamp of 1954 (RW21), the text was printed on top of the gum. The number appears on the back of the selvage adjoining one stamp per sheet of 112 stamps. Each sheet comprises four panes of 28 stamps each, as shown in Figure 1. Reading left to right and top to bottom, the back plate number appears to the right of position 24 of the upper-right pane of 28 stamps (arrow at right in Figure 1). The back plate number is so obscure and scarce that most collectors are unaware of its existence. The 2000 Durland Standard Plate Number Catalog notes the back number in the listings for Scott RW13-18, but it does not price plate blocks with the back number. The Scott Specialized Catalogue of United States Stamps and Covers does not mention the back number at all. Although plate No. 47510 was used on RW13-20, only RW13-17 actually show the back number in the selvage. Beginning with the $2 Gadwall Ducks stamp of 1951 (RW18), the portion of the selvage bearing the back number was trimmed off when the sheets were cut into individual panes. I have seen one misperforated Gadwall Ducks stamp that bears about 30 percent of the number in the selvage, but I've seen no others. The position of a standard back-numbered block of six in a full sheet is indicated by the shading in Figure 1, right. Figure 2 shows the back of a block of six $1 Redhead Ducks stamps. Note the back number on the middle selvage at left. The front of this block is pictured in Figure 3. Recently, a collector asked me how many of these back-numbered blocks exist. It is a good question, and I don't have an exact answer. But after 30 years of studying duck stamps, I believe the back numbers are as scarce as some of the early federal duck plate blocks. Recent auction realizations for selvage-attached stamps or blocks bearing the numbers were generally in the range of $425-$700. Because these numbers are so scarce, I believe they should bring prices in the neighborhood of $2,500. But because many collectors don't understand or are unaware of them, they go relatively unchallenged in the bidding process. An oddity is that singles with attached back-number selvage fetch about $250, making back-numbered blocks selling at the top of the range a bargain. Another possibility is that collectors simply do not care about the plate number printed on the back. To observe it, one must mount the stamp or block gum side up, a rather dull presentation. My gut feeling is that duck stamps with a plate number on the back are not more popular because they are not listed in the Scott catalogs, and they are not included in the Scott duck stamp plate-block album. Of course, unusual items mounted on blank pages can add some pizzazz to a standard album. A complete pane of an early federal duck stamp is very unusual - most collectors have never seen one. An often-puzzling area is the existence of straightedges on the early duck stamps, namely RW1-12. Beginning with RW13 perforations exist on all four sides on all water-activated duck stamps. Beginning with the $3 Labrador and Mallard stamp of 1959 (RW26), duck stamps were printed on a rotary press and issued in panes of 30, a format used through 1999. All previous federal duck stamps (RW1-12) were printed on a flat-bed press, as diagramed in Figure 1. A 20-stamp pane format was adopted for lick-and-stick duck stamps in 2000 (RW67). The flat-press sheet layout in Figure 1 shows the positions of the straightedge stamps ("SE" in the diagram) and the four plate-number blocks of six, as well as the back-numbered block of six. Duck panes from rotary-press printings yield plate blocks of four with the plate number in the corner. An exception is the $3 Hawaiian Nene Geese stamp of 1964 (RW31). Here the arrangement of the plate numbers is similar to a flat-press printing, so a plate block has six stamps instead of four. Take a look at the positions of the straightedge stamps, as shown in the Figure 1 diagram. Of the 28 stamps in a pane, 10 of them (35.7 percent) have a straightedge. Given that many collectors prefer mint, never-hinged stamps with perforations all around, and that an estimated 92 percent of early duck stamps were used by hunters, and that many, many of the remaining unused examples are hinged or have poor centering, one can easily understand the high values for early mint, never-hinged stamps with very fine or better centering. For example, 635,001 of the $1 Mallards stamp (RW1) were sold. Of these, an estimated 584,201 were sold to hunters, leaving 50,800 for collectors, of which 18,135 are straightedge stamps. This leaves 32,665 with perforations on all four sides. Most of these (75 percent, by my estimate) were hinged, leaving 8,166 mint stamps with perforations all around. If only mint stamps with extra fine or better centering are considered, the number of available examples drops precipitously. Now consider that, although the total quantity sold is the third lowest of all federal duck stamps, the $1 Mallards stamp has the highest mint and unused values in the Scott U.S. specialized catalog, and in most other catalogs. The quantities sold of both the $1 Canvasbacks (RW2) and the $1 Canada Geese (RW3) are lower, so theoretically they should have higher catalog values, but they do not. Because most duck stamp collectors collect sets, it would seem that the sheer number of stamps sold would force the scarcest stamps to have the highest values. However, the $1 Mallards stamp is priced much higher because it garners significant collector interest, a great example of demand dictating higher prices. Conversely, lack of demand and inattention from catalog editors can push prices down, as shown by the back-numbered blocks of RW13-17 discussed here. Collectors should be wary of examples of straightedge RW1-12 stamps that have been reperforated. Be wary, too, of early unsigned duck stamps that have been soaked from hunting licenses and regummed. Both circumstances can be difficult to detect and expertizing is suggested.