In volume 4 of R.H. White’s Encyclopedia of the Colors of United States Postage Stamps — still the most comprehensive work on the color of U.S. postage stamps to 1919 — there is a reference to a technical report published by Philatelic Research Ltd. We have not been able to find this report in any philatelic library, or even any other mention of it. If you know anything about this report (see the reference below, from volume 4, page 13 of White’s Encyclopedia), please let us know in the comments!
R.H. White, Tech. Report ENC-IV-9, 1980, Philatelic Research Ltd., Germantown, Maryland.