Thanks again for all the responses.
The washers I received from the supplier in Canada, a large British bike parts outlet, are part number 62302/A washer,rubber/rubber,tap. They are copper on three sides, filled with rubber with the rubber contacting the threads, when crushed the rubber would squeeze into the threads forming a seal.
I can't find it in any of the Norton parts books but it may be another Brit bike part.
It looks to me like some sort of crush washer as it's quite thick, I won't use them as I'm afraid the force required to crush the washer hence making the seal could pull the tap out of the tank.
I went to a Kawasaki dealer today and picked up two of the Stat-O-Seal washers some posters mentioned and are shown in the link supplied by one poster. They said they aren't used on any Kawasaki bikes but had them is stock from working on "odd ball bikes" to quote the dealer.
Again, thanks for all the informative posts.