More to confuse the planet. The aluminum itself is soft and gives to a certain amount to seal better. Ever seen aluminum drain plug washers? I would likely put the copper against the bottom steel of the nut as the steel will NOT be giving to seal there. You generally seal with a hard surface next to a soft one that gives in malleablility. So, aluminum/steel/copper/steel there. I thought those washers were rounded at the stamping but I see so many I could be totally full of crap.
Yes, I know, it's exactly opposed to the copper washers used next to the aluminum of the oil pan at the arrows. But those have to seal against the steel of bolts too.
It really shouldn't matter which side goes where if one uses like Permatex #2 or silicone spread over the washer and nut in a fashion to get into the lows to seal as the bolt or nut tightens up. Then wipe around the joint to clean up any that'll fall loose later. The whole idea is to stop oil leaks, there's no secret 'torque holding' thing or other going on there.
See? I got a answer for everything......................