In an article I wrote that appeared in the November-December 2008 issue of The Collectors Club Philatelist, I discussed the unusual method used to print the 1906 Escalon issue of El Salvador. (This same method was used for the 1907 National Palace issue, the 1910 Figueroa issue and the 1912 Centenary issue.)
The method used to produce the bi-colored, engraved stamps was unusual since there were not separate plates for the frame and the vignette, which was normal for bi-colored engraved stamps at this time. Instead, chalk was put into the vignette and the frame printed, then the chalk was cleaned out of the vignette and put into the frame and the vignette printed. Sometimes this process was reversed.
Production problems sometimes resulted in parts of the design printed in both the color of the vignette and the frame.
Recently, I acquired a group of the postal stationery envelopes of the Escalon issue and upon examining them, I noticed that the printing on one of them show the same doubling in both colors. This, I believe, is strong evidence that the same method was used to print the stationery as well as the adhesives.
Shown here is the envelope as well as an enlargement of the portion of the design with parts of the name being printed in both colors.