A major difference between European and American philately is how collectors in those areas view the signing of stamps. Signing stamps on the back, usually with a handstamp and usually in ink, began in the late Nineteenth Century in Germany as a dealer identification marking. Dealers would sign the stamps that they sold and such signatures, by prestigious companies such as the Senf Brothers, became an early form of guarantee. If Senf sold it, the stamp was genuine and the new owner could buy and sell the stamp confident that he was dealing with the genuine article. This signature system has evolved to the current European system of expert signatures to the point where nearly every European stamp of value has been indelibly marked in ink by various experts to confirm genuineness and provenance.
Americans have always felt this system to be somewhat daft. If the goal in collecting is to find a specimen as close to post office issued quality as possible then the marking of stamps with ink to increase their value by proving genuineness doesn’t make much sense. We have evolved a far more extensive system of photo certificates in this country (and created it far earlier) than they have in Europe. But perhaps the difference is more philatelic than temperamental. By 1900 in Europe, forgeries were a major problem and before inexpensive photography a signature from a renown expert was the easiest way a collector had to set himself at ease that the stamp he was buying was genuine. Counterfeits on US stamps were much less of a problem until about 1930 (by which time photography was far cheaper) when counterfeit coils began flooding the market. This was because strongly enforced counterfeiting laws made knock offs of US stamps too risky whereas the major political transformations of Germany and Italy in the late Nineteenth Century made for hundreds of rare demonetized stamps which no government cared if anyone copied. Such a situation required a cheap and efficient way of proving genuineness and the system of signing stamps fit the bill.