Authenticating Philippines Overprint Stamps

I recently bought three Philippines overprints, PH 223A, 237, 238 (MH). I have time to have them authenticated but am wondering how often these are forged with a fake overprint, especially for the 237 and 238. These stamps are valued by scott at 300 and 550 respectively, however the US version (311 and 312) are valued at 600 and 825 respectively so it seems it wouldn't make sense to apply a forged overprint to these. The 223A is different as it is valued at 2,000 and the US version (276A) is valued at 1,250. I couldn't find any information on line. Thanks

Forgery/fake info.

http://www.philippinephilatelist.ne...s/Page1.html

There is a book by John M. Hunt on the Forged overprints of the US Administration time period. You are correct that the catalogue value of most stamps works against the forger. Try to compare yours to lower values and look for differences.

The populations of real and fake overprints are both pretty low so it's hard to say if the fakes are common or not. What is common is reperfing of both, as for the similar US issues, so be aware of that when buying.

Comparison with the low values you have is your best bet. The experts would take faulty stamps and cut through half of the overprint horizontally to overlay on a stamp to be checked. That would allow comparison of ink, letter thickness and letter spacing.

Thanks, great advice