This is the part
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You could save all round and buy the one on the forum from a looooooongtime member for a great price. If your lucky you may find the rider's backrest that go for about US$400 used these days. The good news is that seat with the hole comes with a rubber bung but works fine without it and is THE most comfortable factory Triumph Roadster or dual headlight seat made and I have owned and tried them all.
[URL unfurl="true"]https://www.r3owners.net/threads/long-haul-dual-seat-and-bracket-for-roadsters.49299/[/URL]
There are a few thread here on the forms posted by me and others on making custom rider's backrests to fit the hole and a number of good to excellent designs exist.
