1908-1922 15¢ Washington
(read how to identify your stamp below)
This is the 15¢ Washington. It is quite common and usually worth no more than a few dollars when used, however it can have reasonable value when unused. As there are only vour variations of this stamp, identification is relatively easy.
There are three variations of this stamp. They are catalog numbers 340, 366 and 382. Catalog number 366 is so rare that you do not need to bother with it. That leaves catalog numbers 340 and 382. The difference between the two is in their watermark. See below.
DETERMINING THE WATERMARK
To know which watermark a stamp has, one needs watermark fluid or lighter fluid (both of these are extremely flammable and should be used with extreme caution, outside, in a safe area far away from combustible materials). Soak the stamp in the fluid in small plate with a black or very dark color. The watermark will show, sometimes faintly by looking at the back of the stamp. The watermark fluid will quickly evaporate from the stamp, leaving the stamp and its gum intact.
There are two types of watermark you will come across. These are;
The watermark will have part or all of the letters U, S or P, as shown above. The upper USP letters are made of double lines, this is known as watermark 191. The lower UPS is made of single lined letters, this is known as watermark 190. Below is a chart indicating the type of watermakring that can be found on each stamp.
RARE PAPER VARITIES
Identifying the type of paper the stamp was printed on is not easy. Usually one does not need to bother with this as over 99% of the 15¢ are printed on regular paper and their price is un-affected by the type of paper. There are two types of papers that the Post Office experimented with in an attempt to save on costs. These are all rare, they are;
China Clay Paper
However, having said all this, certificates are issued for these (on what basis I have no idea), and they do sell at a premium. According to Scotts the paper was accidentally given a high mineral content (its aluminum) and the paper is thick, hard and grayish, often darker than "bluish" paper. The differences are, shall we say, subtle. My advise is not to buy even a certificated version, as Scotts has little supporting evidence to have awarded this category of paper catalog status.