Promoting new color?
As promised here are a few images of the 1918 Harley Accessory Catalog. This catalog is 115 plus pages of everything from touch up paint to sidecar trunks!. I am guessing by the envelope art and the color of the cover that they were promoting the new O.D. color. I'm not sure if this is the year they changed from Silent "Gray" Fellow to Olive Drab.
The envelope also indicates 2-18 (guessing February) and the recipients cycle, an 18 J (S for side car??).
This piece is in new condition. I don't know how this stuff made it this far!