Originally, all philatelic expertization was done by individuals who expertized stamps and covers in their own name usually as part of the professional service of selling them. The most famous three European experts of the nineteenth century- Gebruder Senf(the Senf brothers) and Kohler in Germany and Thier in France were all merchant experts who signed the stamps that they sold and guaranteed the genuineness of their sale material. The American expert dealer Warren H Colson (who signed stamps W.H.C.) and Eugene Klein of Philadelphia provided much the same services in this country in the 1890-1930 period (Klein was the American dealer who was one of the big buyers at the famous Ferrari auctions in Paris in the early 1920s). In those days a dealer guaranteeing the genuineness of the stamps he sold forever was considered the norm and most collectors were far happier having a Klein guarantee than an APS certificate. A Klein guarantee actually meant something. If the stamp was not genuine Eugene Klein would give you your money back.
As philately got more complex and as collectors began to question the neutrality of experts passing on his own stamps, independent experts gave way to committees. Herbert Bloch,the last of the great multicountry individual experizers who died in 1987, once told me that he never greatly pursued the stamp selling part of his stamp business because he felt that it interfered with his expertizing business. Collectors and dealers alike, he felt, preferred him to not actively sell stamps-dealers because they didn’t want their expertizing done by a competitor and collectors because they doubted the impartiality of an active stamp seller. Today nearly all expertizing is done by committee, often semi anonymously. It proves that Bloch’s point about active dealers and expertizing is still valid today that one of the few solo expertizers- William Weiss (who does a pretty good job) only began issuing certificates after he had retired from a successful career as a philatelic auctioneer.