On the PF website they have counterfeits and genuine. Here are images of the counterfeits
And here is image of a genuine overprint
If the PF certifies a stamp as counterfeit the fee is less than the price of a tank of gas. I've had a few stamps come back as counterfeits. Sometimes peace of mind is worth the fee. A stamp like that, if genuine, would need a certificate from a recognized expert in order to be offered at auction. I can not distinguish between the genuine and the counterfeits.