First digit of the serial number is usually the last digit of the year.
Just one problem. The radio was built in Taiwan from 1978 to 1989.
A serial number with a first digit of "8" might be 1978 or 1988.
A "9" might be 1979 or 1989.
Only way to resolve this ambiguity is a peek inside at the date code on chips inside.
The MB8734 PLL chip is usually open-dated, with a four-digit code. Two digits for the year, followed by two digits for the week of that year, 01-52.
A date code of "8301" would indicate January of 1983.
But ONLY if the PLL hasn't be replaced with a MB8719. The radio was never built with that chip, but changing the factory-original MB8734 to a MB8719 is a popular way to expand the frequency coverage. Not the only way, just a popular one.
If the radio has a MB8719, that date code tells you NOTHING about the age of the radio.
Sometimes the "S042" transmit-mixer chip on the left edge of the main circuit board will have this 4-digit open-date code on it.
A peek inside the clock/counter shield will reveal similar 4-digit date codes on some, but not all the logic chips inside.
So long as this module has not been removed and swapped for a different one, this will help pin down the production year.
And if the country of origin is Philippines, the radio was made in 1989 or later, and the same "first-digit" rule should apply.
73