Talking of Dowty washers I'm unsure that we are all talking about the same thing
A Dowty washer as I understand them to be, is a solid steel washer with a rubber section bonded onto the internal circumference of the washer, the rubber section being thicker than the washer makes the seal.
There are also washers that consist of a thickish rubber washer surrounded around it's outer circumference and partly over both side with copper, these I think seal by squishing up the copper ( which the rubber allows )
Both types work fine but I think the first is a "Dowty" washer.
other opinions are available, and no doubt forthcoming