It is always better to have the original certificates in hand. If the certificates are not recent, there is a good possibility that the expertizer put a very small mark on the back of the stamp using indelible ink. This will usually allow you to at least know who expertized the stamp, even if you don't have the certificate in hand.
If marked on the back of the stamp, the expertizer's mark may be as small as 1 mm, or it may be several mm; so you may need to use a magnifier to see it clearly.
However, expertizer marks have been forged, so it will be the original certificate that counts.

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