might be a male mould to then make a female mould from a different material (would have been moulding sand/silica* etc in my day). *bonded sand, oil sand also comes up
It's not a huge picture either: 82 cm high by 60 cm wide. The mirror on the back wall is only about 5" wide on the canvas yet it's a detailed picture within a picture. The brush used must have been just a single hair.
I once watched an amazing documentary which went through all the details of that painting and another similar one.