I think that the aluminium washer is probably a dowty washer, I.e. has a bonded rubber inner and should be changed when removed. Copper would be better in this application
The dowty washers I've seen have always had a steel base. Simply because it doesn't distort under compression and so compromise the rubber seal supported by it. I have seen plenty of plain aluminium sealing washers though with sewin's explanation of their introduction making sense.
