Here's some information for you. Since your unit is a SL1210MK2, it may be harder to determine the age of the unit:
It is possible to tell the year (and the month) when the SL-1200 was
manufactured from the serial number on the back or bottom of the
turntable. SL-1200 serial numbers are in two different formats,
depending on the date of manufacture. It isn't exactly possible to
determine the decade in which the SL-1200 in question was manufactured,
as the serial number contains only the last digit of the year. If the
year digit is 8, the turntable may have been manufactured in 1978, 1988
or 1998. If the year digit is 0, the turntable may have been
manufactured in 1980, 1990 or 2000. One rule of thumb is that
old-format serial numbers belong to SL-1200s manufactured during the
1970s and 1980s, while new-format serial numbers belong to SL-1200s
manufactured during the 1990s and beyond.
Old-format serial number:
NHOJF20765 (SL-1200 manufactured 1982)
The first digit shows the year when the SL-1200 was manufactured. These
serial numbers contain no indication of the month of manufacture. It
should be noted that serial numbers in this format contain no letters
New-format serial number:
GE4FB001154 (SL-1200 manufactured June 2004)
The first digit shows the year when the SL-1200 was manufactured, while
the month of manufacture is indicated by the letter following it.
Months are coded as letters between A and L inclusive; A indicates
January, B indicates February, and so on.
In addition to the above, all the SL-1200MK2s from the 70's and
early 80's have a 4 inch diameter plate where the RCA and ground wires
enter the chassis in the rubber base of the unit. The later models have
a smaller hole through which the wires enter the unit.
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