Drilling the holes in the marble is easily farmed out. There are people who cut holes in marble, granite and other stone for making sinks and countertops. Give them a template and they use a high-pressure water jet to do the dirty work.
Some enthusiasts have mounted their turntables on plinths made of slate. They have the cutout and the tonearm mounting hole, if applicable, cut out the same way. The advantage is mass; the same amount of vibrational energy generated by a turntable motor and bearings will dissipate more effectively in a massive plinth, compared to a lightweight wooden box; and slate is much heavier than the same volume of plywood, making for a smaller slate plinth for the same mass than stacked layers of plywood.
Whether suppression of vibration was the goal of this amplifier, or pure cosmetics, I can't tell. If cosmetics were the only reason, why the dull black original finish on the transformers?