This is an old listing for the single arm plinth, very hard to tell from the pictures the exact construction though.
https://www.canuckaudiomart.com/details/649200296-victor-jvc-cl-p10-turntable-plinth/
And this quote from The Vintageknob on the ql7
"Victor’s CL-P bases are made of four layers of high-density particle boards and three layers of special inorganic material ; I don’t know if that structure was retained for the export black base"
However not sure the total accuracy there.
http://www.thevintageknob.org/jvc-QL-7.html
https://www.canuckaudiomart.com/details/649200296-victor-jvc-cl-p10-turntable-plinth/
And this quote from The Vintageknob on the ql7
"Victor’s CL-P bases are made of four layers of high-density particle boards and three layers of special inorganic material ; I don’t know if that structure was retained for the export black base"
However not sure the total accuracy there.
http://www.thevintageknob.org/jvc-QL-7.html