The ET-number identifies the PCB. It is not the serial number. The serial number can be found stamped (4 digits) or on a sticker (6 digits) in the battery compartment.
A "Made in Japan" SD-1 is definitely 80s. Prices differ wildly. A paid around $25 for mine a month ago while a nearby store tries to sell one for $110. I guess the average is somewhere in the middle.