A certified copy is a copy/reproduction of the original document. So technically when someone in authority signed a document as certified true copy of the original, it becomes valid only at the time it was signed and confirmed by the said authority. Don't have much knowledge about the seal, all I know is that there are lots of them along recto joke
Kidding aside, I guess the best way for the verifying party to prove the validity of a certified true copy document is through contacting the issuing institution...nothing less...so if resigned na yun signatory definitely di masasagot nung currently in the position e di naman sya ang pumirma e