This metal is just to add mass ?
Not exactly,
The idea of building up the plinth from laminated plywood, with or without a steel plate was to produce a dense non resonant base for the bearing / platter and armboard, making the plinth with shot filled cavities would imo compromise the design.
I understand the reasons behind filling stands / speakers with shot or sand, but with a deck that seems like locking the stable door after the horse has bolted, (only necessary due to bad design)
I think we can make a less resonant deck with far better isolation from vibration via the original route. The reason I wanted to add the metal plate was to provide an enhanced interface between the bearing and the plinth. It seems pointless to ruin a quality bearing by screwing it onto plywood. By bolting the bearing to the metal you can guarantee a superior fixing, and by then laminating that into the plywood you end up with a more substantial structure, the bearing and lower plinth become in effect one piece, along with the added benefit of loads of mass. If we can make the whole deck as one unit (plus the compliant layer) and it has enough mass, then if mounted on sorborthane or similar feet it will be impervious to vibration.
I wait to be shot down in flames