Nice stamp, no gum. Determined genuine. The certificate is of the opinion 

that the overprint was partially removed and has a fake cancellation.

I would propose looking closely and judging for yourself. 

The missing part of the overprint is exactly where 

the jacket is printed, perhaps the ink from the overprint 

was not able to adhere to that part of the stamp and 

the other dark areas might be a smearing of residual ink.