Bumping this older thread. I'd thought I'd revive it since we left with an open question as to whether they were fakes. I picked up some "normal" stamps a year ago, and finally got around to them. The only one that helps me is Congo Democratic Republic Scott 327, which is the 50c on 60c revalue. Here is the "normal" stamp.

Observant people may notice a slight ghost Congo overprint at the left bottom, but that is not the issue at hand.
When I take the 327 "invert" and flip it over, here is what I get:

When I zoom into the revalue section on the invert, I see the following:

And here is that same overprint on the "normal":

The main difference that I can see is the spacing between the 0 of 50 and the cent symbol is slightly wider on the "normal" stamp. The bottom of the C also has a slight "hook". Any other comments?