You need a standard saddle clamp - the sort of thing you still find on cheap mountain bikes and childrens bikes. They are available in
Halfords or probably any decent bike shop (or your nearest skip - have a look). It will look like this
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I don't think the number of rails will have any real effect. Very old saddles were all twin or triple railed. The B66 is twin railed and I think Brooks brought out the B67 as a single rail version of the B66 purely so people could use it on a modern style seatpin