This is what came with the SP1023 - non-G - bulkhead fitting from Amazon, in a sealed Hayward plastic bag: a bulkhead fitting, 1 backing nut, 2 rubber washers and 1 cork washer.

I'm not going to use the cork washer at all. I will use both rubber washers, one inside and the other outside. I don't think the outside will be necessary for sealing, but it will provide a cushion between the vinyl liner and the hard edge of the backing nut. I don't think there would be any advantage to a sandwich washer for me. If one had 2 layers, where the vinyl liner was supported by a metal or fiberglass shell, the sandwich washer might be useful in ensuring water couldn't get BETWEEN the layers.