French Colonies Overprints - Forgeries?

Hi - going through some long forgotten lots and came across these French Offices in China overprints. The first is definitely different than the other 2 in text width while the second seems not to be straight. I suspect a couple of these are forgeries. The second photo is from a Sphinx Auction showing a pair of 5 FR's which I assume is authentic (they didn't list a cert so perhaps I'm being naïve saying this is a genuine reference).

I can't seem to locate a good website with help identifying forgeries of this type ... anyone know of a good place to study this or care to throw out an opinion of the stamps shown? Thanks.

They look genuine to me. Different types exist of the 'CHINE' surcharge.The narrow & wide 'C'types. Your first two stamps(Narrow) and the third stamp (Wide).Also the 'H' can be found a bit wider like in your 5F stamp.
Many of these surcharges were done under private contracts with the French administration.Control was not always optimal and clandestine surcharges were done for the collectors market.
Readable and dated cancels make these stamps more interesting IMO.
An nice web site about early Indochina Philately(in French).

http://www.franceasie19.sitew.fr/Philatelie.f.htm

Thanks for the link perf12. That history is very interesting. Thank goodness for Google translate!

Overprint forgery usually occurs when the regular type is worth pennies and the overprinted one big money -- eg the early Berlin issues.

Only an idiot would ruin that quite valuable France 5F stamp by sticking a fake overprint on it.

Generally, overprinting is quite a low-tech procedure and the quality control is not very high.