Serial number
Serial numbers for Sigma guitars built in South Korea, Taiwan and Indonesia are perhaps even more meaningless, though again some seem to indicate the year of manufacture (e.g. loop81XXXXXX.) Since we know these instruments were manufactured from 1984 through 2007 in these three countries, it is safe to assume that a serial number beginning with 81XXXXXX (which I have personally seen) were not produced in 1981 and that these serial number 'indicators' are not to be trusted.