It was assayed in a foreign country. Back in the days of silver & gold standard you could get rich transferring precious metal coins from where the silver or gold market was soft to where it was strong. U.S. coins were not pure silver, but some other countries were. China, for one, was sharp enough to limit the influx of foreign currency and they had to have a "chop" mark. I believe Spain and many other countries did the same. When vendors saw the assay stamp they could instantly figure the value of foreign currency by weight alone.