Hi,
Here is a link to the service manual for the turntable.
According to the parts diagram in the manual, the counter weight part number is X-4857-606-0.
(click on image to enlarge for better viewing)
Unfortunately, I have been unable to locate the part online by using the part number in the browser’s search term. You may have better luck. You may have to contact a turntable accessories supplier or Sony in your location to see if they can get one for you.
If you wish to download the manual when the sentence below the Document preview box on the webpage changes from This file is downloadable free of charge: ...processing... to This file is downloadable free of charge: Download, click on Download as this is the link. Be patient, it takes a little while to find the file.
Hopefully this is of some help
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