I grinned the surface down by hand, to bear-metal and as I suspected/hoped – I found raw aluminum (Al 13).
The top-coat of some sort of plastic was hiding the share beauty of undiluted and pure metal.
I used two different grains of paper P80 and P120.
I do wish I would have owned a hand-controlled electric tool of some sort or even better my own shop.
It would have facilitated matters considerably.