Expertizing committees such as the PF, APEX, PSE, and others have been around for a long time. They offer certificates of authenticity, usually accompanied with a photo of the item that was expertized. Collectors depend on the reputations of these committees to know that they are buying accurately described material.
Sometimes you’ll see a stamp or cover offered as “signed by XXX.” Where the “XXX” is some famous philatelist, like Stanley Ashbrook. Before there were expertizing committees, prominent collectors were called upon to render judgment on an item. When they did that, they would sign their name very small on the item. That was their stamp of authenticity. In some cases, some collectors used a tiny rubber stamp instead.
This was in the 1930s and 1940s. Before there were photocopy machines, computers, and the sophisticated technology that expertizing committees use today. Of course, signatures are easy to fake. And with any fake, there is a possibility of financial gain. Fakers would add signatures to their items to give them providence as having been examined by a famous philatelist.
Years ago, collectors would pay more for an item if it was signed (expertized) by some prominent philatelist. Their signature was equivalent to today’s certificate.
Even today, signatures on stamps will carry a premium, even if they don’t have a certificate. Because some collectors want to own a stamp that was handled by some prominent philatelist.