See below for details
8¢ - Olive green, deep olive green
Printing Method: FLAT PLATE
Subject: George Washington
Number issued: 94,000,000
Perforations: P12
Watermark: Double Line USPS
Scott #: 338
Issued: December 12, 1908
Used
$2 - $4
No postmark with gum (MH)
$10 - $30
Full perfect gum, no postmark
no trace of stamp hinge mark (MNH)
$80 - $120
#337 was issued with the following plate #'s
Imprint and number
4919-24
Sometimes you will see very expensive copies of #333 advertised as experimental paper or CHINA CLAY PAPER. There is no such thing, what exists are stamps which have "dirty water" paper because silica in the water became embedded in paper produced during drought conditions when the mill pond was low.
There are some old PF certificates still around that state that a stamp is a "China Clay' stamp. The PF no longer recognises China Clay stamps and has ceased certifying them.
Do not be tempted to buy something that does not exist.
The US was a member of the Universal Postal Union (UPU) and under the terms of the agreement each member was required to send a copy of a new design to all the Universal Postal Union countries. Today, most countries still have these samples. Portugal does not, it distributed its samples long ago. Portugal overprinted all their samples with the word 'Ultramar' which translates to 'Specimen'. For the whole of the period of the production of the Washington Franklin series only one example of each value was sent to Portugal, therefore finding one is not easy.
Finding #337 on cover is hard. There are very few survivors. Here is an example, an odd one at that. You will notice that it has two different postmarks, one from 1912 and another from 1915.
Bust of George Washington
by Jean-Antoine Houdon
1785
The source of the design
Die proof on India mounted on card
337-P1