This countermark does not appear in Brunk's book, but most such countermarks remain unlisted. Yours looks like it could be the mark of an early smith, silversmith, gunsmith, blacksmith or toolmaker, given its crude style within a toothed border. The style looks early and the coin was probably stamped not many years after it began circulating. Spanish coins circulated freely here, and this counterstamp is probably of U.S. origin. Attribution may be possible, but it's like finding the proverbial needle in a haystack. On
ebay, it might fetch $30-$40 as a curiosity, I suspect. The larger Spanish coins, particularly the 2 reale pieces were more often hosts to merchant counterstamps. Neat item!