Malaria Philatelists International (MPI)

The February 2009 Miasma had the article Look At The Back Of Your Covers. I was looking at some duplicates to sell to someone and I thought I would listen to my own words - and it is a good thing that I did. Below are two FDCs that I looked at the back and found stamps cancelled on the back. Apparently, the rate to mail the covers to the United States was more than the the single stamp alone. These are both addressed to Ben Waltzer in New York. I wonder if there is a complete set of 15 from the 15 French Colonies with stamps on teh back to make up the correct rate? I am also not sure why the Niger FDC was cancelled April 13th on the back instead of April 7th.