I only have this pair, but I see that Stanley Gibbons notes:
"The overprint settings consisted of mixed type and many varieties exist, including mixed numerals, closed '4', wrong font 'C' in 'Cameroun', broken 'B' for 'R', etc."
Scott 240, SG 164. In my example, the 27.8.40 definitely has a different 2 and 4. Any other examples out there?