Our best historian... Worked here since the 70's doesn't not know for sure, but also thinks it is a mold indication.
He mentioned that hulls were made in many places around the world as well.
if this is the case, how were the very old 16's given their three digit serial numbers( stamped on the pylons ) were the issued by hobie america or could there be more than one boat with the same three digit serial number in other countries?
I would think it would depend on the country's regulations regarding serial numbers at the time (if there were any). That's what prompted the change in 1973 from 4-digit serial numbers on the pylons (which are presumably sequential) to the CCM####M##X format in the USA that identifies the manufacturer (CCM - Coast Catamaran), the serial number (####) and the model year & month of manufacture (M##X). The format is similar today.