The third digit of the serial number indicates the year built. If it is indeed a zero, the unit was made in 1990 or maybe 1980 regardless of what HifiEngine says. Could that zero be an 8 missing the top loop or middle line?That digit is correct, Pete. The first number (5) is set off a bit from the rest, in case that makes a difference.
HifiEngine lists manufacturing years as 87-88.