Hi
The definitive book for these issues is:
Colombia 1904-1917: Types of the 'Numeral' Issues by John N. Myer. I have seen this on
ebay once or twice. Mr. Myer has written a number of books on early Colombia:
The 1866 Issue of Colombia
Colombia 1904-1917: Types of the 'Numeral' Issues
Colombia - The 25c of 1869 on Pink Paper
The "Honda" Provisional of Tolima (Colombia)
The Second Issue of Airmail in Colombia
Studies in the Philately of Colombia
One can search the APRL (American Philatelic Research Library)
with
John N. Myer to get re-prints of these articles:
http://www.stamps.org/inmagicgenie/...RY&Type=opac If not an APS member one would have to use the Interlibrary service of their own library to get a reprint.
http://www.stamplibrary.org/thelibr...ces.htm#feesJerry B