I can't find the error you mention in the Neefus Handbook for any of the surcharged 50 marku stamps. There is an error (Mi210 XVI for the 15 c on 50 m, Mi218 XIV for 25 c on 50 m, Mi226 XIII for 30 c in 50 m) described as "the right value box and the "5" have colour dots. This doesn't seem to be what you show here, which looks quite ordinary apart from some small inking irregularity in the lower bar and the flag of the '5'.
Tim, thanks for responding. The stamp is 15c on 50m. The flaw in question is on the original stamp (Mi.145 PF IV), not the overprint, and so it would appear on all the surcharged varieties as well. Michel lists it as 210 PF XIV Farbklecks uber "5" der rechten Wertangabe "50", so I'm pretty sure we're talking about the same flaw. I have a PDF of the Neefus handbook, where it's also listed as 210 PF XIV. The small inking irregularity in the flag of the "5" that you mentioned is what I'm thinking constitutes the plate flaw. It may be mostly obscured by the upper bar. Also, there is a very small dot inside the "0", and this might be part of the same flaw. Ideally, I'm hoping that someone who has seen this plate flaw can verify if it's supposed to look like my stamp.
Thanks for clarifying. Unfortunately I don't have this plate flaw so I can't give an example. I don't think many folk on this forum collect these stamps, but if anyone does, please have a look if you have this flaw and show us an example.
I checked my stamp to see if it has the break above the "R" like on the above photo of 145 IV (as Tim most alertly points out). The overprint also hides this area, but it appears there is some kind of break there, only a little different. So I'm pretty sure I don't have sheet III, position 78. I guess the whole issue could be plated fairly easily and there could be many more "plate flaws" than those listed in Michel or the Neefus Handbook, depending on how minor you're willing to go.
Also, Tim, is there really a stamp that just says "Africa" on it? Seems a little non-specific!