period correct aluminium plates?

If you want reflective alloy plates then as long as they conform to the current regulations (otherwise risk MOT fail) then fit whatever you want.
Such alloy plates were / are more expensive than the more common plastic types and that is the only reason I can think of why they went out of fashion. A dealer could have the equipment to make plastic plates a lot cheaper that the press to make alloy. I've had alloy black & white plates and mixed results. They had to be made to order and I found the black is a separate paper thin section of alloy that is stuck to the bare pressed alloy. On one set this black came loose and started to lift off so the replacement were traditional pressed alloy with black paint - but were a lot more expensive. I suspect pressed reflective plates will be made in the same way as the duff pair I first had with a stuck on background so say make sure you use a reputable supplier and not cheapest off eBay.