I ran across 2 of these inverts while transferring Somali Coast collection. Presuming the inverts are from what Maury calls "tirages clandestins" (illegal printings) in Paris.
First row is 1902 issue, shows Sc #41 normal and then invert, also #45
Second row is 1903 issue (black centres), show normal Sc #61, inverted #62
I don't have any background on it, but below is what Maury says on differences between those ones and legit. If anyone does have more detail, would love to hear it. Didn't see anything mentioned at stampforgeries.com beyond a reference to an article by the French & Colonies Philatelic Society journal from 1943 (which I don't have obv.).